Optimizer Hints
TiDB supports optimizer hints, which are based on the comment-like syntax introduced in MySQL 5.7. For example, one of the common syntaxes is /*+ HINT_NAME([t1_name [, t2_name] ...]) */. Use of optimizer hints is recommended in cases where the TiDB optimizer selects a less optimal query plan.
If you encounter a situation where hints do not take effect, see Troubleshoot common issues that hints do not take effect.
Syntax
Optimizer hints are case insensitive and specified within /*+ ... */ comments following the SELECT, INSERT, UPDATE or DELETE keyword in a SQL statement.
Multiple hints can be specified by separating with commas. For example, the following query uses three different hints:
SELECT /*+ USE_INDEX(t1, idx1), HASH_AGG(), HASH_JOIN(t1) */ count(*) FROM t t1, t t2 WHERE t1.a = t2.b;
How optimizer hints affect query execution plans can be observed in the output of EXPLAIN and EXPLAIN ANALYZE.
An incorrect or incomplete hint will not result in a statement error. This is because hints are intended to have only a hint (suggestion) semantic to query execution. Similarly, TiDB will at most return a warning if a hint is not applicable.
Currently, TiDB supports two categories of hints, which are different in scope. The first category of hints takes effect in the scope of query blocks, such as /*+ HASH_AGG() */; the second category of hints takes effect in the whole query, such as /*+ MEMORY_QUOTA(1024 MB)*/.
Each query or sub-query in a statement corresponds to a different query block, and each query block has its own name. For example:
SELECT * FROM (SELECT * FROM t) t1, (SELECT * FROM t) t2;
The above query statement has three query blocks: the outermost SELECT corresponds to the first query block, whose name is sel_1; the two SELECT sub-queries correspond to the second and the third query block, whose names are sel_2 and sel_3, respectively. The sequence of the numbers is based on the appearance of SELECT from left to right. If you replace the first SELECT with DELETE or UPDATE, then the corresponding query block names are del_1 or upd_1.
Hints that take effect in query blocks
This category of hints can follow behind any SELECT, UPDATE or DELETE keywords. To control the effective scope of the hint, use the name of the query block in the hint. You can make the hint parameters clear by accurately identifying each table in the query (in case of duplicated table names or aliases). If no query block is specified in the hint, the hint takes effect in the current block by default.
For example:
SELECT /*+ HASH_JOIN(@sel_1 t1@sel_1, t3) */ * FROM (SELECT t1.a, t1.b FROM t t1, t t2 WHERE t1.a = t2.a) t1, t t3 WHERE t1.b = t3.b;
This hint takes effect in the sel_1 query block, and its parameters are the t1 and t3 tables in sel_1 (sel_2 also contains a t1 table).
As described above, you can specify the name of the query block in the hint in the following ways:
- Set the query block name as the first parameter of the hint, and separate it from other parameters with a space. In addition to
QB_NAME, all the hints listed in this section also have another optional hidden parameter@QB_NAME. By using this parameter, you can specify the effective scope of this hint. - Append
@QB_NAMEto a table name in the parameter to explicitly specify which query block this table belongs to.
QB_NAME
If the query statement is a complicated statement that includes multiple nested queries, the ID and name of a certain query block might be mistakenly identified. The hint QB_NAME can help us in this regard.
QB_NAME means Query Block Name. You can specify a new name to a query block. The specified QB_NAME and the previous default name are both valid. For example:
SELECT /*+ QB_NAME(QB1) */ * FROM (SELECT * FROM t) t1, (SELECT * FROM t) t2;
This hint specifies the outer SELECT query block's name to QB1, which makes QB1 and the default name sel_1 both valid for the query block.
MERGE_JOIN(t1_name [, tl_name ...])
The MERGE_JOIN(t1_name [, tl_name ...]) hint tells the optimizer to use the sort-merge join algorithm for the given table(s). Generally, this algorithm consumes less memory but takes longer processing time. If there is a very large data volume or insufficient system memory, it is recommended to use this hint. For example:
select /*+ MERGE_JOIN(t1, t2) */ * from t1, t2 where t1.id = t2.id;
NO_MERGE_JOIN(t1_name [, tl_name ...])
The NO_MERGE_JOIN(t1_name [, tl_name ...]) hint tells the optimizer not to use the sort-merge join algorithm for the given table(s). For example:
SELECT /*+ NO_MERGE_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
INL_JOIN(t1_name [, tl_name ...])
The INL_JOIN(t1_name [, tl_name ...]) hint tells the optimizer to use the index nested loop join algorithm for the given table(s). This algorithm might consume less system resources and take shorter processing time in some scenarios and might produce an opposite result in other scenarios. If the result set is less than 10,000 rows after the outer table is filtered by the WHERE condition, it is recommended to use this hint. For example:
SELECT /*+ INL_JOIN(t1, t2) */ * FROM t1, t2, t3 WHERE t1.id = t2.id AND t2.id = t3.id;
In the preceding SQL statement, the INL_JOIN(t1, t2) hint tells the optimizer to use the index nested loop join algorithm for t1 and t2. Note that this does not mean that the index nested loop join algorithm is used between t1 and t2. Instead, the hint indicates that t1 and t2 each use the index nested loop join algorithm with another table (t3).
The parameter(s) given in INL_JOIN() is the candidate table for the inner table when you create the query plan. For example, INL_JOIN(t1) means that TiDB only considers using t1 as the inner table to create a query plan. If the candidate table has an alias, you must use the alias as the parameter in INL_JOIN(); if it does not have an alias, use the table's original name as the parameter. For example, in the select /*+ INL_JOIN(t1) */ * from t t1, t t2 where t1.a = t2.b; query, you must use the t table's alias t1 or t2 rather than t as INL_JOIN()'s parameter.
NO_INDEX_JOIN(t1_name [, tl_name ...])
The NO_INDEX_JOIN(t1_name [, tl_name ...]) hint tells the optimizer not to use the index nested loop join algorithm for the given table(s). For example:
SELECT /*+ NO_INDEX_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
INL_HASH_JOIN
The INL_HASH_JOIN(t1_name [, tl_name]) hint tells the optimizer to use the index nested loop hash join algorithm. The conditions for using this algorithm are the same with the conditions for using the index nested loop join algorithm. The difference between the two algorithms is that INL_JOIN creates a hash table on the joined inner table, but INL_HASH_JOIN creates a hash table on the joined outer table. INL_HASH_JOIN has a fixed limit on memory usage, while the memory used by INL_JOIN depends on the number of rows matched in the inner table.
NO_INDEX_HASH_JOIN(t1_name [, tl_name ...])
The NO_INDEX_HASH_JOIN(t1_name [, tl_name ...]) hint tells the optimizer not to use the index nested loop hash join algorithm for the given table(s).
INL_MERGE_JOIN
The INL_MERGE_JOIN(t1_name [, tl_name]) hint tells the optimizer to use the index nested loop merge join algorithm. The conditions for using this algorithm are the same with the conditions for using the index nested loop join algorithm.
NO_INDEX_MERGE_JOIN(t1_name [, tl_name ...])
The NO_INDEX_MERGE_JOIN(t1_name [, tl_name ...]) hint tells the optimizer not to use the index nested loop merge join algorithm for the given table(s).
HASH_JOIN(t1_name [, tl_name ...])
The HASH_JOIN(t1_name [, tl_name ...]) hint tells the optimizer to use the hash join algorithm for the given table(s). This algorithm allows the query to be executed concurrently with multiple threads, which achieves a higher processing speed but consumes more memory. For example:
select /*+ HASH_JOIN(t1, t2) */ * from t1, t2 where t1.id = t2.id;
NO_HASH_JOIN(t1_name [, tl_name ...])
The NO_HASH_JOIN(t1_name [, tl_name ...]) hint tells the optimizer not to use the hash join algorithm for the given table(s). For example:
SELECT /*+ NO_HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
HASH_JOIN_BUILD(t1_name [, tl_name ...])
The HASH_JOIN_BUILD(t1_name [, tl_name ...]) hint tells the optimizer to use the hash join algorithm on specified tables with these tables working as the build side. In this way, you can build hash tables using specific tables. For example:
SELECT /*+ HASH_JOIN_BUILD(t1) */ * FROM t1, t2 WHERE t1.id = t2.id;
HASH_JOIN_PROBE(t1_name [, tl_name ...])
The HASH_JOIN_PROBE(t1_name [, tl_name ...]) hint tells the optimizer to use the hash join algorithm on specified tables with these tables working as the probe side. In this way, you can execute the hash join algorithm with specific tables as the probe side. For example:
SELECT /*+ HASH_JOIN_PROBE(t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
SEMI_JOIN_REWRITE()
The SEMI_JOIN_REWRITE() hint tells the optimizer to rewrite the semi-join query to an ordinary join query. Currently, this hint only works for EXISTS subqueries.
If this hint is not used to rewrite the query, when the hash join is selected in the execution plan, the semi-join query can only use the subquery to build a hash table. In this case, when the result of the subquery is bigger than that of the outer query, the execution speed might be slower than expected.
Similarly, when the index join is selected in the execution plan, the semi-join query can only use the outer query as the driving table. In this case, when the result of the subquery is smaller than that of the outer query, the execution speed might be slower than expected.
When SEMI_JOIN_REWRITE() is used to rewrite the query, the optimizer can extend the selection range to select a better execution plan.
-- Does not use SEMI_JOIN_REWRITE() to rewrite the query.
EXPLAIN SELECT * FROM t WHERE EXISTS (SELECT 1 FROM t1 WHERE t1.a = t.a);
+-----------------------------+---------+-----------+------------------------+---------------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+---------+-----------+------------------------+---------------------------------------------------+
| MergeJoin_9 | 7992.00 | root | | semi join, left key:test.t.a, right key:test.t1.a |
| ├─IndexReader_25(Build) | 9990.00 | root | | index:IndexFullScan_24 |
| │ └─IndexFullScan_24 | 9990.00 | cop[tikv] | table:t1, index:idx(a) | keep order:true, stats:pseudo |
| └─IndexReader_23(Probe) | 9990.00 | root | | index:IndexFullScan_22 |
| └─IndexFullScan_22 | 9990.00 | cop[tikv] | table:t, index:idx(a) | keep order:true, stats:pseudo |
+-----------------------------+---------+-----------+------------------------+---------------------------------------------------+
-- Uses SEMI_JOIN_REWRITE() to rewrite the query.
EXPLAIN SELECT * FROM t WHERE EXISTS (SELECT /*+ SEMI_JOIN_REWRITE() */ 1 FROM t1 WHERE t1.a = t.a);
+------------------------------+---------+-----------+------------------------+---------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+------------------------------+---------+-----------+------------------------+---------------------------------------------------------------------------------------------------------------+
| IndexJoin_16 | 1.25 | root | | inner join, inner:IndexReader_15, outer key:test.t1.a, inner key:test.t.a, equal cond:eq(test.t1.a, test.t.a) |
| ├─StreamAgg_39(Build) | 1.00 | root | | group by:test.t1.a, funcs:firstrow(test.t1.a)->test.t1.a |
| │ └─IndexReader_34 | 1.00 | root | | index:IndexFullScan_33 |
| │ └─IndexFullScan_33 | 1.00 | cop[tikv] | table:t1, index:idx(a) | keep order:true |
| └─IndexReader_15(Probe) | 1.25 | root | | index:Selection_14 |
| └─Selection_14 | 1.25 | cop[tikv] | | not(isnull(test.t.a)) |
| └─IndexRangeScan_13 | 1.25 | cop[tikv] | table:t, index:idx(a) | range: decided by [eq(test.t.a, test.t1.a)], keep order:false, stats:pseudo |
+------------------------------+---------+-----------+------------------------+---------------------------------------------------------------------------------------------------------------+
From the preceding example, you can see that when using the SEMI_JOIN_REWRITE() hint, TiDB can select the execution method of IndexJoin based on the driving table t1.
SHUFFLE_JOIN(t1_name [, tl_name ...])
The SHUFFLE_JOIN(t1_name [, tl_name ...]) hint tells the optimizer to use the Shuffle Join algorithm on specified tables. This hint only takes effect in the MPP mode. For example:
SELECT /*+ SHUFFLE_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
BROADCAST_JOIN(t1_name [, tl_name ...])
BROADCAST_JOIN(t1_name [, tl_name ...]) hint tells the optimizer to use the Broadcast Join algorithm on specified tables. This hint only takes effect in the MPP mode. For example:
SELECT /*+ BROADCAST_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
NO_DECORRELATE()
The NO_DECORRELATE() hint tells the optimizer not to try to perform decorrelation for the correlated subquery in the specified query block. This hint is applicable to the EXISTS, IN, ANY, ALL, SOME subqueries and scalar subqueries that contain correlated columns (that is, correlated subqueries).
When this hint is used in a query block, the optimizer will not try to perform decorrelation for the correlated columns between the subquery and its outer query block, but always use the Apply operator to execute the query.
By default, TiDB tries to perform decorrelation for correlated subqueries to achieve higher execution efficiency. However, in some scenarios, decorrelation might actually reduce the execution efficiency. In this case, you can use this hint to manually tell the optimizer not to perform decorrelation. For example:
create table t1(a int, b int);
create table t2(a int, b int, index idx(b));
-- Not using NO_DECORRELATE().
explain select * from t1 where t1.a < (select sum(t2.a) from t2 where t2.b = t1.b);
+----------------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+----------------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------+
| HashJoin_11 | 9990.00 | root | | inner join, equal:[eq(test.t1.b, test.t2.b)], other cond:lt(cast(test.t1.a, decimal(10,0) BINARY), Column#7) |
| ├─HashAgg_23(Build) | 7992.00 | root | | group by:test.t2.b, funcs:sum(Column#8)->Column#7, funcs:firstrow(test.t2.b)->test.t2.b |
| │ └─TableReader_24 | 7992.00 | root | | data:HashAgg_16 |
| │ └─HashAgg_16 | 7992.00 | cop[tikv] | | group by:test.t2.b, funcs:sum(test.t2.a)->Column#8 |
| │ └─Selection_22 | 9990.00 | cop[tikv] | | not(isnull(test.t2.b)) |
| │ └─TableFullScan_21 | 10000.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
| └─TableReader_15(Probe) | 9990.00 | root | | data:Selection_14 |
| └─Selection_14 | 9990.00 | cop[tikv] | | not(isnull(test.t1.b)) |
| └─TableFullScan_13 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+----------------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------+
From the preceding execution plan, you can see that the optimizer has automatically performed decorrelation. The decorrelated execution plan does not have the Apply operator. Instead, the plan has join operations between the subquery and the outer query block. The original filter condition (t2.b = t1.b) with the correlated column becomes a regular join condition.
-- Using NO_DECORRELATE().
explain select * from t1 where t1.a < (select /*+ NO_DECORRELATE() */ sum(t2.a) from t2 where t2.b = t1.b);
+------------------------------------------+-----------+-----------+------------------------+--------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+------------------------------------------+-----------+-----------+------------------------+--------------------------------------------------------------------------------------+
| Projection_10 | 10000.00 | root | | test.t1.a, test.t1.b |
| └─Apply_12 | 10000.00 | root | | CARTESIAN inner join, other cond:lt(cast(test.t1.a, decimal(10,0) BINARY), Column#7) |
| ├─TableReader_14(Build) | 10000.00 | root | | data:TableFullScan_13 |
| │ └─TableFullScan_13 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
| └─MaxOneRow_15(Probe) | 10000.00 | root | | |
| └─StreamAgg_20 | 10000.00 | root | | funcs:sum(Column#14)->Column#7 |
| └─Projection_45 | 100000.00 | root | | cast(test.t2.a, decimal(10,0) BINARY)->Column#14 |
| └─IndexLookUp_44 | 100000.00 | root | | |
| ├─IndexRangeScan_42(Build) | 100000.00 | cop[tikv] | table:t2, index:idx(b) | range: decided by [eq(test.t2.b, test.t1.b)], keep order:false, stats:pseudo |
| └─TableRowIDScan_43(Probe) | 100000.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
+------------------------------------------+-----------+-----------+------------------------+--------------------------------------------------------------------------------------+
From the preceding execution plan, you can see that the optimizer does not perform decorrelation. The execution plan still contains the Apply operator. The filter condition (t2.b = t1.b) with the correlated column is still the filter condition when accessing the t2 table.
HASH_AGG()
The HASH_AGG() hint tells the optimizer to use the hash aggregation algorithm in all the aggregate functions in the specified query block. This algorithm allows the query to be executed concurrently with multiple threads, which achieves a higher processing speed but consumes more memory. For example:
select /*+ HASH_AGG() */ count(*) from t1, t2 where t1.a > 10 group by t1.id;
STREAM_AGG()
The STREAM_AGG() hint tells the optimizer to use the stream aggregation algorithm in all the aggregate functions in the specified query block. Generally, this algorithm consumes less memory but takes longer processing time. If there is a very large data volume or insufficient system memory, it is recommended to use this hint. For example:
select /*+ STREAM_AGG() */ count(*) from t1, t2 where t1.a > 10 group by t1.id;
MPP_1PHASE_AGG()
MPP_1PHASE_AGG() tells the optimizer to use the one-phase aggregation algorithm for all aggregate functions in the specified query block. This hint only takes effect in the MPP mode. For example:
SELECT /*+ MPP_1PHASE_AGG() */ COUNT(*) FROM t1, t2 WHERE t1.a > 10 GROUP BY t1.id;
MPP_2PHASE_AGG()
MPP_2PHASE_AGG() tells the optimizer to use the two-phase aggregation algorithm for all aggregate functions in the specified query block. This hint only takes effect in the MPP mode. For example:
SELECT /*+ MPP_2PHASE_AGG() */ COUNT(*) FROM t1, t2 WHERE t1.a > 10 GROUP BY t1.id;
USE_INDEX(t1_name, idx1_name [, idx2_name ...])
The USE_INDEX(t1_name, idx1_name [, idx2_name ...]) hint tells the optimizer to use only the given index(es) for a specified t1_name table. For example, applying the following hint has the same effect as executing the select * from t t1 use index(idx1, idx2); statement.
SELECT /*+ USE_INDEX(t1, idx1, idx2) */ * FROM t1;
FORCE_INDEX(t1_name, idx1_name [, idx2_name ...])
The FORCE_INDEX(t1_name, idx1_name [, idx2_name ...]) hint tells the optimizer to use only the given index(es).
The usage and effect of FORCE_INDEX(t1_name, idx1_name [, idx2_name ...]) are the same as the usage and effect of USE_INDEX(t1_name, idx1_name [, idx2_name ...]).
The following 4 queries have the same effect:
SELECT /*+ USE_INDEX(t, idx1) */ * FROM t;
SELECT /*+ FORCE_INDEX(t, idx1) */ * FROM t;
SELECT * FROM t use index(idx1);
SELECT * FROM t force index(idx1);
IGNORE_INDEX(t1_name, idx1_name [, idx2_name ...])
The IGNORE_INDEX(t1_name, idx1_name [, idx2_name ...]) hint tells the optimizer to ignore the given index(es) for a specified t1_name table. For example, applying the following hint has the same effect as executing the select * from t t1 ignore index(idx1, idx2); statement.
select /*+ IGNORE_INDEX(t1, idx1, idx2) */ * from t t1;
ORDER_INDEX(t1_name, idx1_name [, idx2_name ...])
The ORDER_INDEX(t1_name, idx1_name [, idx2_name ...]) hint tells the optimizer to use only the given index for a specified table and read the specified index in order.
This hint is usually applied in the following scenario:
CREATE TABLE t(a INT, b INT, key(a), key(b));
EXPLAIN SELECT /*+ ORDER_INDEX(t, a) */ a FROM t ORDER BY a LIMIT 10;
+----------------------------+---------+-----------+---------------------+-------------------------------+
| id | estRows | task | access object | operator info |
+----------------------------+---------+-----------+---------------------+-------------------------------+
| Limit_10 | 10.00 | root | | offset:0, count:10 |
| └─IndexReader_14 | 10.00 | root | | index:Limit_13 |
| └─Limit_13 | 10.00 | cop[tikv] | | offset:0, count:10 |
| └─IndexFullScan_12 | 10.00 | cop[tikv] | table:t, index:a(a) | keep order:true, stats:pseudo |
+----------------------------+---------+-----------+---------------------+-------------------------------+
The optimizer generates two types of plan for this query: Limit + IndexScan(keep order: true) and TopN + IndexScan(keep order: false). When the ORDER_INDEX hint is used, the optimizer chooses the first plan that reads the index in order.
NO_ORDER_INDEX(t1_name, idx1_name [, idx2_name ...])
The NO_ORDER_INDEX(t1_name, idx1_name [, idx2_name ...]) hint tells the optimizer to use only the given index for a specified table and not to read the specified index in order. This hint is usually applied in the following scenario.
The following example shows that the effect of the query statement is equivalent to SELECT * FROM t t1 use index(idx1, idx2);:
CREATE TABLE t(a INT, b INT, key(a), key(b));
EXPLAIN SELECT /*+ NO_ORDER_INDEX(t, a) */ a FROM t ORDER BY a LIMIT 10;
+----------------------------+----------+-----------+---------------------+--------------------------------+
| id | estRows | task | access object | operator info |
+----------------------------+----------+-----------+---------------------+--------------------------------+
| TopN_7 | 10.00 | root | | test.t.a, offset:0, count:10 |
| └─IndexReader_14 | 10.00 | root | | index:TopN_13 |
| └─TopN_13 | 10.00 | cop[tikv] | | test.t.a, offset:0, count:10 |
| └─IndexFullScan_12 | 10000.00 | cop[tikv] | table:t, index:a(a) | keep order:false, stats:pseudo |
+----------------------------+----------+-----------+---------------------+--------------------------------+
The same as the example of ORDER_INDEX hint, the optimizer generates two types of plans for this query: Limit + IndexScan(keep order: true) and TopN + IndexScan(keep order: false). When the NO_ORDER_INDEX hint is used, the optimizer will choose the latter plan to read the index out of order.
AGG_TO_COP()
The AGG_TO_COP() hint tells the optimizer to push down the aggregate operation in the specified query block to the coprocessor. If the optimizer does not push down some aggregate function that is suitable for pushdown, then it is recommended to use this hint. For example:
select /*+ AGG_TO_COP() */ sum(t1.a) from t t1;
LIMIT_TO_COP()
The LIMIT_TO_COP() hint tells the optimizer to push down the Limit and TopN operators in the specified query block to the coprocessor. If the optimizer does not perform such an operation, it is recommended to use this hint. For example:
SELECT /*+ LIMIT_TO_COP() */ * FROM t WHERE a = 1 AND b > 10 ORDER BY c LIMIT 1;
READ_FROM_STORAGE(TIFLASH[t1_name [, tl_name ...]], TIKV[t2_name [, tl_name ...]])
The READ_FROM_STORAGE(TIFLASH[t1_name [, tl_name ...]], TIKV[t2_name [, tl_name ...]]) hint tells the optimizer to read specific table(s) from specific storage engine(s). Currently, this hint supports two storage engine parameters - TIKV and TIFLASH. If a table has an alias, use the alias as the parameter of READ_FROM_STORAGE(); if the table does not has an alias, use the table's original name as the parameter. For example:
select /*+ READ_FROM_STORAGE(TIFLASH[t1], TIKV[t2]) */ t1.a from t t1, t t2 where t1.a = t2.a;
USE_INDEX_MERGE(t1_name, idx1_name [, idx2_name ...])
The USE_INDEX_MERGE(t1_name, idx1_name [, idx2_name ...]) hint tells the optimizer to access a specific table with the index merge method. Index merge has two types: intersection type and union type. For details, see Explain Statements Using Index Merge.
If you explicitly specify the list of indexes, TiDB selects indexes from the list to build index merge; if you do not specify the list of indexes, TiDB selects indexes from all available indexes to build index merge.
For the intersection-type index merge, the given list of indexes is a required parameter in the hint. For the union-type index merge, the given list of indexes is an optional parameter in the hint. See the following example.
SELECT /*+ USE_INDEX_MERGE(t1, idx_a, idx_b, idx_c) */ * FROM t1 WHERE t1.a > 10 OR t1.b > 10;
When multiple USE_INDEX_MERGE hints are made to the same table, the optimizer tries to select the index from the union of the index sets specified by these hints.
LEADING(t1_name [, tl_name ...])
The LEADING(t1_name [, tl_name ...]) hint reminds the optimizer that, when generating the execution plan, to determine the order of multi-table joins according to the order of table names specified in the hint. For example:
SELECT /*+ LEADING(t1, t2) */ * FROM t1, t2, t3 WHERE t1.id = t2.id and t2.id = t3.id;
In the above query with multi-table joins, the order of joins is determined by the order of table names specified in the LEADING() hint. The optimizer will first join t1 and t2 and then join the result with t3. This hint is more general than STRAIGHT_JOIN.
The LEADING hint does not take effect in the following situations:
- Multiple
LEADINGhints are specified. - The table name specified in the
LEADINGhint does not exist. - A duplicated table name is specified in the
LEADINGhint. - The optimizer cannot perform join operations according to the order as specified by the
LEADINGhint. - The
straight_join()hint already exists. - The query contains an outer join together with the Cartesian product.
In the preceding situations, a warning is generated.
-- Multiple `LEADING` hints are specified.
SELECT /*+ LEADING(t1, t2) LEADING(t3) */ * FROM t1, t2, t3 WHERE t1.id = t2.id and t2.id = t3.id;
-- To learn why the `LEADING` hint fails to take effect, execute `show warnings`.
SHOW WARNINGS;
+---------+------+-------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------------------------------------------------------+
| Warning | 1815 | We can only use one leading hint at most, when multiple leading hints are used, all leading hints will be invalid |
+---------+------+-------------------------------------------------------------------------------------------------------------------+
MERGE()
Using the MERGE() hint in queries with common table expressions (CTE) can disable the materialization of the subqueries and expand the subquery inlines into CTE. This hint is only applicable to non-recursive CTE. In some scenarios, using MERGE() brings higher execution efficiency than the default behavior of allocating a temporary space. For example, pushing down query conditions or in nesting CTE queries:
-- Uses the hint to push down the predicate of the outer query.
WITH CTE AS (SELECT /*+ MERGE() */ * FROM tc WHERE tc.a < 60) SELECT * FROM CTE WHERE CTE.a < 18;
-- Uses the hint in a nested CTE query to expand a CTE inline into the outer query.
WITH CTE1 AS (SELECT * FROM t1), CTE2 AS (WITH CTE3 AS (SELECT /*+ MERGE() */ * FROM t2), CTE4 AS (SELECT * FROM t3) SELECT * FROM CTE3, CTE4) SELECT * FROM CTE1, CTE2;
Hints that take effect globally
The global hint works in views. When specified as a global hint, the hint defined in a query can take effect inside the view. To specify a global hint, first use the QB_NAME hint to define a query block name, and then add the target hints in the form of ViewName@QueryBlockName.
Step 1: Define the query block name of the view using the QB_NAME hint
Use the QB_NAME hint to define a new name for each query block of the view. The definition of the QB_NAME hint for views is the same as that for query blocks, but the syntax is extended from QB_NAME(QB) to QB_NAME(QB, ViewName@QueryBlockName [.ViewName@QueryBlockName .ViewName@QueryBlockName ...]).
For a simple statement with a single view and no subqueries, the following example specifies the first query block name of view
v:SELECT /* Comment: The name of the current query block is the default @SEL_1 */ * FROM v;For view
v, the first view name in the list (ViewName@QueryBlockName [.ViewName@QueryBlockName .ViewName@QueryBlockName ...]) starting from the query statement isv@SEL_1. The first query block of the viewvcan be declared asQB_NAME(v_1, v@SEL_1 .@SEL_1), or simply written asQB_NAME(v_1, v), omitting@SEL_1:CREATE VIEW v AS SELECT /* Comment: The name of the current query block is the default @SEL_1 */ * FROM t; -- Specifies the global hint SELECT /*+ QB_NAME(v_1, v) USE_INDEX(t@v_1, idx) */ * FROM v;For a complex statement with nested views and subqueries, the following example specifies the names for each of two query blocks of the view
v1andv2:SELECT /* Comment: The name of the current query block is the default @SEL_1 */ * FROM v2 JOIN ( SELECT /* Comment: The name of the current query block is the default @SEL_2 */ * FROM v2) vv;For the first view
v2, the first view name in the list starting from the first query statement isv2@SEL_1. For the second viewv2, the first view name isv2@SEL_2. The following example only considers the first viewv2.The first query block of view
v2can be declared asQB_NAME(v2_1, v2@SEL_1 .@SEL_1), and the second query block of the viewv2can be declared asQB_NAME(v2_2, v2@SEL_1 .@SEL_2):CREATE VIEW v2 AS SELECT * FROM t JOIN /* Comment: For view v2, the name of the current query block is the default @SEL_1. So, the current query block view list is v2@SEL_1 .@SEL_1 */ ( SELECT COUNT(*) FROM t1 JOIN v1 /* Comment: For view v2, the name of the current query block is the default @SEL_2. So, the current query block view list is v2@SEL_1 .@SEL_2 */ ) tt;For view
v1, the first view name in the list starting from the preceding statement isv2@SEL_1 .v1@SEL_2. The first query block in viewv1can be declared asQB_NAME(v1_1, v2@SEL_1 .v1@SEL_2 .@SEL_1), and the second query block in viewv1can be declared asQB_NAME(v1_2, v2@SEL_1 .v1@SEL_2 .@SEL_2):CREATE VIEW v1 AS SELECT * FROM t JOIN /* Comment: For view `v1`, the name of the current query block is the default @SEL_1. So, the current query block view list is v2@SEL_1 .@SEL_2 .v1@SEL_1 */ ( SELECT COUNT(*) FROM t1 JOIN t2 /* Comment: For view `v1`, the name of the current query block is the default @SEL_2. So, the current query block view list is v2@SEL_1 .@SEL_2 .v1@SEL_2 */ ) tt;
Step 2: Add the target hints
After defining the QB_NAME hint for query blocks of the view, you can add required hints that take effect in query blocks in the form of ViewName@QueryBlockName to make them effective inside the view. For example:
Specify the
MERGE_JOIN()hint for the first query block of the viewv2:SELECT /*+ QB_NAME(v2_1, v2) merge_join(t@v2_1) */ * FROM v2;Specify the
MERGE_JOIN()andSTREAM_AGG()hints for the second query block of the viewv2:SELECT /*+ QB_NAME(v2_2, v2.@SEL_2) merge_join(t1@v2_2) stream_agg(@v2_2) */ * FROM v2;Specify the
HASH_JOIN()hint for the first query block of the viewv1:SELECT /*+ QB_NAME(v1_1, v2.v1@SEL_2) hash_join(t@v1_1) */ * FROM v2;Specify the
HASH_JOIN()andHASH_AGG()hints for the second query block of the viewv1:SELECT /*+ QB_NAME(v1_2, v2.v1@SEL_2 .@SEL_2) hash_join(t1@v1_2) hash_agg(@v1_2) */ * FROM v2;
Hints that take effect in the whole query
This category of hints can only follow behind the first SELECT, UPDATE or DELETE keyword, which is equivalent to modifying the value of the specified system variable when this query is executed. The priority of the hint is higher than that of existing system variables.
NO_INDEX_MERGE()
The NO_INDEX_MERGE() hint disables the index merge feature of the optimizer.
For example, the following query will not use index merge:
select /*+ NO_INDEX_MERGE() */ * from t where t.a > 0 or t.b > 0;
In addition to this hint, setting the tidb_enable_index_merge system variable also controls whether to enable this feature.
USE_TOJA(boolean_value)
The boolean_value parameter can be TRUE or FALSE. The USE_TOJA(TRUE) hint enables the optimizer to convert an in condition (containing a sub-query) to join and aggregation operations. Comparatively, the USE_TOJA(FALSE) hint disables this feature.
For example, the following query will convert in (select t2.a from t2) subq to corresponding join and aggregation operations:
select /*+ USE_TOJA(TRUE) */ t1.a, t1.b from t1 where t1.a in (select t2.a from t2) subq;
In addition to this hint, setting the tidb_opt_insubq_to_join_and_agg system variable also controls whether to enable this feature.
MAX_EXECUTION_TIME(N)
The MAX_EXECUTION_TIME(N) hint places a limit N (a timeout value in milliseconds) on how long a statement is permitted to execute before the server terminates it. In the following hint, MAX_EXECUTION_TIME(1000) means that the timeout is 1000 milliseconds (that is, 1 second):
select /*+ MAX_EXECUTION_TIME(1000) */ * from t1 inner join t2 where t1.id = t2.id;
In addition to this hint, the global.max_execution_time system variable can also limit the execution time of a statement.
MEMORY_QUOTA(N)
The MEMORY_QUOTA(N) hint places a limit N (a threshold value in MB or GB) on how much memory a statement is permitted to use. When a statement's memory usage exceeds this limit, TiDB produces a log message based on the statement's over-limit behavior or just terminates it.
In the following hint, MEMORY_QUOTA(1024 MB) means that the memory usage is limited to 1024 MB:
select /*+ MEMORY_QUOTA(1024 MB) */ * from t;
In addition to this hint, the tidb_mem_quota_query system variable can also limit the memory usage of a statement.
READ_CONSISTENT_REPLICA()
The READ_CONSISTENT_REPLICA() hint enables the feature of reading consistent data from the TiKV follower node. For example:
select /*+ READ_CONSISTENT_REPLICA() */ * from t;
In addition to this hint, setting the tidb_replica_read environment variable to 'follower' or 'leader' also controls whether to enable this feature.
IGNORE_PLAN_CACHE()
The IGNORE_PLAN_CACHE() hint reminds the optimizer not to use the Plan Cache when handling the current prepare statement.
This hint is used to temporarily disable the Plan Cache for a certain type of queries when prepare-plan-cache is enabled.
In the following example, the Plan Cache is forcibly disabled when executing the prepare statement.
prepare stmt from 'select /*+ IGNORE_PLAN_CACHE() */ * from t where t.id = ?';
SET_VAR(VAR_NAME=VAR_VALUE)
You can temporarily modify the value of system variables during statement execution by using the SET_VAR(VAR_NAME=VAR_VALUE) hint. After the statement is executed, the value of the system variable in the current session is automatically restored to the original value. This hint can be used to modify some system variables related to the optimizer and executor. For a list of system variables that can be modified using this hint, refer to System Variables.
The following is an example:
SELECT /*+ SET_VAR(MAX_EXECUTION_TIME=1234) */ @@MAX_EXECUTION_TIME;
SELECT @@MAX_EXECUTION_TIME;
After executing the preceding SQL statements, the first query returns the value 1234 set in the hint, instead of the default value of MAX_EXECUTION_TIME. The second query returns the default value of the variable.
+----------------------+
| @@MAX_EXECUTION_TIME |
+----------------------+
| 1234 |
+----------------------+
1 row in set (0.00 sec)
+----------------------+
| @@MAX_EXECUTION_TIME |
+----------------------+
| 0 |
+----------------------+
1 row in set (0.00 sec)
STRAIGHT_JOIN()
The STRAIGHT_JOIN() hint reminds the optimizer to join tables in the order of table names in the FROM clause when generating the join plan.
SELECT /*+ STRAIGHT_JOIN() */ * FROM t t1, t t2 WHERE t1.a = t2.a;
NTH_PLAN(N)
The NTH_PLAN(N) hint reminds the optimizer to select the Nth physical plan found during the physical optimization. N must be a positive integer.
If the specified N is beyond the search range of the physical optimization, TiDB will return a warning and select the optimal physical plan based on the strategy that ignores this hint.
This hint does not take effect when the cascades planner is enabled.
In the following example, the optimizer is forced to select the third physical plan found during the physical optimization:
SELECT /*+ NTH_PLAN(3) */ count(*) from t where a > 5;
RESOURCE_GROUP(resource_group_name)
RESOURCE_GROUP(resource_group_name) is used for Resource Control to isolate resources. This hint temporarily executes the current statement using the specified resource group. If the specified resource group does not exist, this hint will be ignored.
Example:
SELECT /*+ RESOURCE_GROUP(rg1) */ * FROM t limit 10;
Troubleshoot common issues that hints do not take effect
Hints do not take effect because your MySQL command-line client strips hints
MySQL command-line clients earlier than 5.7.7 strip optimizer hints by default. If you want to use the Hint syntax in these earlier versions, add the --comments option when starting the client. For example: mysql -h 127.0.0.1 -P 4000 -uroot --comments.
Hints do not take effect because the database name is not specified
If you do not specify the database name when creating a connection, hints might not take effect. For example:
When connecting to TiDB, you use the mysql -h127.0.0.1 -P4000 -uroot command without the -D option, and then execute the following SQL statements:
SELECT /*+ use_index(t, a) */ a FROM test.t;
SHOW WARNINGS;
Because TiDB cannot identify the database for table t, the use_index(t, a) hint does not take effect.
+---------+------+----------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------------------------------------+
| Warning | 1815 | use_index(.t, a) is inapplicable, check whether the table(.t) exists |
+---------+------+----------------------------------------------------------------------+
1 row in set (0.00 sec)
Hints do not take effect because the database name is not explicitly specified in cross-table queries
When executing cross-table queries, you need to explicitly specify database names. Otherwise, hints might not take effect. For example:
USE test1;
CREATE TABLE t1(a INT, KEY(a));
USE test2;
CREATE TABLE t2(a INT, KEY(a));
SELECT /*+ use_index(t1, a) */ * FROM test1.t1, t2;
SHOW WARNINGS;
In the preceding statements, because table t1 is not in the current test2 database, the use_index(t1, a) hint does not take effect.
+---------+------+----------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------------------------------------------------+
| Warning | 1815 | use_index(test2.t1, a) is inapplicable, check whether the table(test2.t1) exists |
+---------+------+----------------------------------------------------------------------------------+
1 row in set (0.00 sec)
In this case, you need to specify the database name explicitly by using use_index(test1.t1, a) instead of use_index(t1, a).
Hints do not take effect because they are placed in wrong locations
Hints cannot take effect if they are not placed directly after the specific keywords. For example:
SELECT * /*+ use_index(t, a) */ FROM t;
SHOW WARNINGS;
The warning is as follows:
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1064 | You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use [parser:8066]Optimizer hint can only be followed by certain keywords like SELECT, INSERT, etc. |
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)
In this case, you need to place the hint directly after the SELECT keyword. For more details, see the Syntax section.
INL_JOIN hint does not take effect
INL_JOIN hint does not take effect when built-in functions are used on columns for joining tables
In some cases, if you use a built-in function on a column that joins tables, the optimizer might fail to choose the IndexJoin plan, resulting in the INL_JOIN hint not taking effect either.
For example, the following query uses the built-in function substr on the column tname that joins tables:
CREATE TABLE t1 (id varchar(10) primary key, tname varchar(10));
CREATE TABLE t2 (id varchar(10) primary key, tname varchar(10));
EXPLAIN SELECT /*+ INL_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id=t2.id and SUBSTR(t1.tname,1,2)=SUBSTR(t2.tname,1,2);
The execution plan is as follows:
+------------------------------+----------+-----------+---------------+-----------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+------------------------------+----------+-----------+---------------+-----------------------------------------------------------------------+
| HashJoin_12 | 12500.00 | root | | inner join, equal:[eq(test.t1.id, test.t2.id) eq(Column#5, Column#6)] |
| ├─Projection_17(Build) | 10000.00 | root | | test.t2.id, test.t2.tname, substr(test.t2.tname, 1, 2)->Column#6 |
| │ └─TableReader_19 | 10000.00 | root | | data:TableFullScan_18 |
| │ └─TableFullScan_18 | 10000.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
| └─Projection_14(Probe) | 10000.00 | root | | test.t1.id, test.t1.tname, substr(test.t1.tname, 1, 2)->Column#5 |
| └─TableReader_16 | 10000.00 | root | | data:TableFullScan_15 |
| └─TableFullScan_15 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+------------------------------+----------+-----------+---------------+-----------------------------------------------------------------------+
7 rows in set, 1 warning (0.01 sec)
SHOW WARNINGS;
+---------+------+------------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+------------------------------------------------------------------------------------+
| Warning | 1815 | Optimizer Hint /*+ INL_JOIN(t1, t2) */ or /*+ TIDB_INLJ(t1, t2) */ is inapplicable |
+---------+------+------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
As you can see from the preceding example, the INL_JOIN hint does not take effect. This is due to a limitation of the optimizer that prevents using the Projection or Selection operator as the probe side of IndexJoin.
Starting from TiDB v8.0.0, you can avoid this issue by setting tidb_enable_inl_join_inner_multi_pattern to ON.
SET @@tidb_enable_inl_join_inner_multi_pattern=ON;
Query OK, 0 rows affected (0.00 sec)
EXPLAIN SELECT /*+ INL_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id=t2.id AND SUBSTR(t1.tname,1,2)=SUBSTR(t2.tname,1,2);
+------------------------------+--------------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+------------------------------+--------------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------+
| IndexJoin_18 | 12500.00 | root | | inner join, inner:Projection_14, outer key:test.t1.id, inner key:test.t2.id, equal cond:eq(Column#5, Column#6), eq(test.t1.id, test.t2.id) |
| ├─Projection_32(Build) | 10000.00 | root | | test.t1.id, test.t1.tname, substr(test.t1.tname, 1, 2)->Column#5 |
| │ └─TableReader_34 | 10000.00 | root | | data:TableFullScan_33 |
| │ └─TableFullScan_33 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
| └─Projection_14(Probe) | 100000000.00 | root | | test.t2.id, test.t2.tname, substr(test.t2.tname, 1, 2)->Column#6 |
| └─TableReader_13 | 10000.00 | root | | data:TableRangeScan_12 |
| └─TableRangeScan_12 | 10000.00 | cop[tikv] | table:t2 | range: decided by [eq(test.t2.id, test.t1.id)], keep order:false, stats:pseudo |
+------------------------------+--------------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------+
7 rows in set (0.00 sec)
INL_JOIN, INL_HASH_JOIN, and INL_MERGE_JOIN hints do not take effect due to collation incompatibility
When the collation of the join key is incompatible between two tables, the IndexJoin operator cannot be utilized to execute the query. In this case, the INL_JOIN, INL_HASH_JOIN, and INL_MERGE_JOIN hints do not take effect. For example:
CREATE TABLE t1 (k varchar(8), key(k)) COLLATE=utf8mb4_general_ci;
CREATE TABLE t2 (k varchar(8), key(k)) COLLATE=utf8mb4_bin;
EXPLAIN SELECT /*+ tidb_inlj(t1) */ * FROM t1, t2 WHERE t1.k=t2.k;
The execution plan is as follows:
+-----------------------------+----------+-----------+----------------------+----------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+----------+-----------+----------------------+----------------------------------------------+
| HashJoin_19 | 12487.50 | root | | inner join, equal:[eq(test.t1.k, test.t2.k)] |
| ├─IndexReader_24(Build) | 9990.00 | root | | index:IndexFullScan_23 |
| │ └─IndexFullScan_23 | 9990.00 | cop[tikv] | table:t2, index:k(k) | keep order:false, stats:pseudo |
| └─IndexReader_22(Probe) | 9990.00 | root | | index:IndexFullScan_21 |
| └─IndexFullScan_21 | 9990.00 | cop[tikv] | table:t1, index:k(k) | keep order:false, stats:pseudo |
+-----------------------------+----------+-----------+----------------------+----------------------------------------------+
5 rows in set, 1 warning (0.00 sec)
In the preceding statements, the collations of t1.k and t2.k are incompatible (utf8mb4_general_ci and utf8mb4_bin respectively), which prevents the INL_JOIN or TIDB_INLJ hint from taking effect.
SHOW WARNINGS;
+---------+------+----------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------------------------------------------+
| Warning | 1815 | Optimizer Hint /*+ INL_JOIN(t1) */ or /*+ TIDB_INLJ(t1) */ is inapplicable |
+---------+------+----------------------------------------------------------------------------+
1 row in set (0.00 sec)
INL_JOIN hint does not take effect due to join order
The INL_JOIN(t1, t2) or TIDB_INLJ(t1, t2) hint semantically instructs t1 and t2 to act as inner tables in an IndexJoin operator to join with other tables, rather than directly joining them using an IndexJoin operator. For example:
EXPLAIN SELECT /*+ inl_join(t1, t3) */ * FROM t1, t2, t3 WHERE t1.id = t2.id AND t2.id = t3.id AND t1.id = t3.id;
+---------------------------------+----------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------------+----------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| IndexJoin_16 | 15625.00 | root | | inner join, inner:TableReader_13, outer key:test.t2.id, test.t1.id, inner key:test.t3.id, test.t3.id, equal cond:eq(test.t1.id, test.t3.id), eq(test.t2.id, test.t3.id) |
| ├─IndexJoin_34(Build) | 12500.00 | root | | inner join, inner:TableReader_31, outer key:test.t2.id, inner key:test.t1.id, equal cond:eq(test.t2.id, test.t1.id) |
| │ ├─TableReader_40(Build) | 10000.00 | root | | data:TableFullScan_39 |
| │ │ └─TableFullScan_39 | 10000.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
| │ └─TableReader_31(Probe) | 10000.00 | root | | data:TableRangeScan_30 |
| │ └─TableRangeScan_30 | 10000.00 | cop[tikv] | table:t1 | range: decided by [test.t2.id], keep order:false, stats:pseudo |
| └─TableReader_13(Probe) | 12500.00 | root | | data:TableRangeScan_12 |
| └─TableRangeScan_12 | 12500.00 | cop[tikv] | table:t3 | range: decided by [test.t2.id test.t1.id], keep order:false, stats:pseudo |
+---------------------------------+----------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
In the preceding example, t1 and t3 are not directly joined together by an IndexJoin.
To perform a direct IndexJoin between t1 and t3, you can first use LEADING(t1, t3) hint to specify the join order of t1 and t3, and then use the INL_JOIN hint to specify the join algorithm. For example:
EXPLAIN SELECT /*+ leading(t1, t3), inl_join(t3) */ * FROM t1, t2, t3 WHERE t1.id = t2.id AND t2.id = t3.id AND t1.id = t3.id;
+---------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------+
| Projection_12 | 15625.00 | root | | test.t1.id, test.t1.name, test.t2.id, test.t2.name, test.t3.id, test.t3.name |
| └─HashJoin_21 | 15625.00 | root | | inner join, equal:[eq(test.t1.id, test.t2.id) eq(test.t3.id, test.t2.id)] |
| ├─TableReader_36(Build) | 10000.00 | root | | data:TableFullScan_35 |
| │ └─TableFullScan_35 | 10000.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
| └─IndexJoin_28(Probe) | 12500.00 | root | | inner join, inner:TableReader_25, outer key:test.t1.id, inner key:test.t3.id, equal cond:eq(test.t1.id, test.t3.id) |
| ├─TableReader_34(Build) | 10000.00 | root | | data:TableFullScan_33 |
| │ └─TableFullScan_33 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
| └─TableReader_25(Probe) | 10000.00 | root | | data:TableRangeScan_24 |
| └─TableRangeScan_24 | 10000.00 | cop[tikv] | table:t3 | range: decided by [test.t1.id], keep order:false, stats:pseudo |
+---------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------+
9 rows in set (0.01 sec)
Using hints causes the Can't find a proper physical plan for this query error
The Can't find a proper physical plan for this query error might occur in the following scenarios:
- A query itself does not require reading indexes in order. That is, for this query, the optimizer does not generate a plan to read indexes in order in any case without using hints. In this case, if the
ORDER_INDEXhint is specified, this error occurs. To resolve this issue, remove the correspondingORDER_INDEXhint. - A query excludes all possible join methods by using the
NO_JOINrelated hints.
CREATE TABLE t1 (a INT);
CREATE TABLE t2 (a INT);
EXPLAIN SELECT /*+ NO_HASH_JOIN(t1), NO_MERGE_JOIN(t1) */ * FROM t1, t2 WHERE t1.a=t2.a;
ERROR 1815 (HY000): Internal : Can't find a proper physical plan for this query
- The system variable
tidb_opt_enable_hash_joinis set toOFF, and all other join types are also excluded.
CREATE TABLE t1 (a INT);
CREATE TABLE t2 (a INT);
set tidb_opt_enable_hash_join=off;
EXPLAIN SELECT /*+ NO_MERGE_JOIN(t1) */ * FROM t1, t2 WHERE t1.a=t2.a;
ERROR 1815 (HY000): Internal : Can't find a proper physical plan for this query
SET_VAR does not take effect when written in subqueries
SET_VAR is used to modify the value of system variables for the current statement. Do not write it in subqueries. If you write it in a subquery, SET_VAR might not take effect due to the special handling of subqueries.
In the following example, SET_VAR is written in the subquery, so it does not take effect.
mysql> SELECT @@MAX_EXECUTION_TIME, a FROM (SELECT /*+ SET_VAR(MAX_EXECUTION_TIME=123) */ 1 as a) t;
+----------------------+---+
| @@MAX_EXECUTION_TIME | a |
+----------------------+---+
| 0 | 1 |
+----------------------+---+
1 row in set (0.00 sec)
In the following example, SET_VAR is not written in a subquery, so it takes effect.
mysql> SELECT /*+ SET_VAR(MAX_EXECUTION_TIME=123) */ @@MAX_EXECUTION_TIME, a FROM (SELECT 1 as a) t;
+----------------------+---+
| @@MAX_EXECUTION_TIME | a |
+----------------------+---+
| 123 | 1 |
+----------------------+---+
1 row in set (0.00 sec)