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Predicates Push Down (PPD)

This document introduces one of the TiDB's logic optimization rules—Predicate Push Down (PPD). It aims to help you understand the predicate push down and know its applicable and inapplicable scenarios.

PPD pushes down selection operators to data source as close as possible to complete data filtering as early as possible, which significantly reduces the cost of data transmission or computation.

Examples

The following cases describe the optimization of PPD. Case 1, 2, and 3 are scenarios where PPD is applicable, and Case 4, 5, and 6 are scenarios where PPD is not applicable.

Case 1: push predicates to storage layer

create table t(id int primary key, a int);
explain select * from t where a < 1;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id                      | estRows  | task      | access object | operator info                  |
+-------------------------+----------+-----------+---------------+--------------------------------+
| TableReader_7           | 3323.33  | root      |               | data:Selection_6               |
| └─Selection_6           | 3323.33  | cop[tikv] |               | lt(test.t.a, 1)                |
|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.00 sec)

In this query, pushing down the predicate a < 1 to the TiKV layer to filter the data can reduce the overhead of network transmission.

Case 2: push predicates to storage layer

create table t(id int primary key, a int not null);
explain select * from t where a < substring('123', 1, 1);
+-------------------------+----------+-----------+---------------+--------------------------------+
| id                      | estRows  | task      | access object | operator info                  |
+-------------------------+----------+-----------+---------------+--------------------------------+
| TableReader_7           | 3323.33  | root      |               | data:Selection_6               |
| └─Selection_6           | 3323.33  | cop[tikv] |               | lt(test.t.a, 1)                |
|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+

This query has the same execution plan as the query in case 1, because the input parameters of the substring of the predicate a < substring('123', 1, 1) are constants, so they can be calculated in advance. Then the predicate is simplified to the equivalent predicate a < 1. After that, TiDB can push a < 1 down to TiKV.

Case 3: push predicates below join operator

create table t(id int primary key, a int not null);
create table s(id int primary key, a int not null);
explain select * from t join s on t.a = s.a where t.a < 1;
+------------------------------+----------+-----------+---------------+--------------------------------------------+
| id                           | estRows  | task      | access object | operator info                              |
+------------------------------+----------+-----------+---------------+--------------------------------------------+
| HashJoin_8                   | 4154.17  | root      |               | inner join, equal:[eq(test.t.a, test.s.a)] |
| ├─TableReader_15(Build)      | 3323.33  | root      |               | data:Selection_14                          |
| │ └─Selection_14             | 3323.33  | cop[tikv] |               | lt(test.s.a, 1)                            |
| │   └─TableFullScan_13       | 10000.00 | cop[tikv] | table:s       | keep order:false, stats:pseudo             |
| └─TableReader_12(Probe)      | 3323.33  | root      |               | data:Selection_11                          |
|   └─Selection_11             | 3323.33  | cop[tikv] |               | lt(test.t.a, 1)                            |
|     └─TableFullScan_10       | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo             |
+------------------------------+----------+-----------+---------------+--------------------------------------------+
7 rows in set (0.00 sec)

In this query, the predicate t.a < 1 is pushed below join to filter in advance, which can reduce the calculation overhead of join.

In addition，This SQL statement has an inner join executed, and the ON condition is t.a = s.a. The predicate s.a <1 can be derived from t.a < 1 and pushed down to s table below the join operator. Filtering the s table can further reduce the calculation overhead of join.

Case 4: predicates that are not supported by storage layers cannot be pushed down

create table t(id int primary key, a int not null);
desc select * from t where substring('123', a, 1) = '1';
+-------------------------+---------+-----------+---------------+----------------------------------------+
| id                      | estRows | task      | access object | operator info                          |
+-------------------------+---------+-----------+---------------+----------------------------------------+
| Selection_7             | 2.00    | root      |               | eq(substring("123", test.t.a, 1), "1") |
| └─TableReader_6         | 2.00    | root      |               | data:TableFullScan_5                   |
|   └─TableFullScan_5     | 2.00    | cop[tikv] | table:t       | keep order:false, stats:pseudo         |
+-------------------------+---------+-----------+---------------+----------------------------------------+

In this query, there is a predicate substring('123', a, 1) = '1'.

From the explain results, we can see that the predicate is not pushed down to TiKV for calculation. This is because the TiKV coprocessor does not support the built-in function substring.

Case 5: predicates of inner tables on the outer join can't be pushed down

create table t(id int primary key, a int not null);
create table s(id int primary key, a int not null);
explain select * from t left join s on t.a = s.a where s.a is null;
+-------------------------------+----------+-----------+---------------+-------------------------------------------------+
| id                            | estRows  | task      | access object | operator info                                   |
+-------------------------------+----------+-----------+---------------+-------------------------------------------------+
| Selection_7                   | 10000.00 | root      |               | isnull(test.s.a)                                |
| └─HashJoin_8                  | 12500.00 | root      |               | left outer join, equal:[eq(test.t.a, test.s.a)] |
|   ├─TableReader_13(Build)     | 10000.00 | root      |               | data:TableFullScan_12                           |
|   │ └─TableFullScan_12        | 10000.00 | cop[tikv] | table:s       | keep order:false, stats:pseudo                  |
|   └─TableReader_11(Probe)     | 10000.00 | root      |               | data:TableFullScan_10                           |
|     └─TableFullScan_10        | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo                  |
+-------------------------------+----------+-----------+---------------+-------------------------------------------------+
6 rows in set (0.00 sec)

In this query，there is a predicate s.a is null on the inner table s.

From the explain results，we can see that the predicate is not pushed below join operator. This is because the outer join fills the inner table with NULL values when the on condition isn't satisfied, and the predicate s.a is null is used to filter the results after the join. If it is pushed down to the inner table below join, the execution plan is not equivalent to the original one.

Case 6: the predicates which contain user variables cannot be pushed down

create table t(id int primary key, a char);
set @a = 1;
explain select * from t where a < @a;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id                      | estRows  | task      | access object | operator info                  |
+-------------------------+----------+-----------+---------------+--------------------------------+
| Selection_5             | 8000.00  | root      |               | lt(test.t.a, getvar("a"))      |
| └─TableReader_7         | 10000.00 | root      |               | data:TableFullScan_6           |
|   └─TableFullScan_6     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.00 sec)

In this query，there is a predicate a < @a on table t. The @a of the predicate is a user variable.

As can be seen from explain results, the predicate is not like case 2, which is simplified to a < 1 and pushed down to TiKV. This is because the value of the user variable @a may change during the computation, and TiKV is not aware of the changes. So TiDB does not replace @a with 1, and does not push down it to TiKV.

An example to help you understand is as follows：

create table t(id int primary key, a int);
insert into t values(1, 1), (2,2);
set @a = 1;
select id, a, @a:=@a+1 from t where a = @a;
+----+------+----------+
| id | a    | @a:=@a+1 |
+----+------+----------+
|  1 |    1 | 2        |
|  2 |    2 | 3        |
+----+------+----------+
2 rows in set (0.00 sec)

As you can see from this query, the value of @a will change during the query. So if you replace a = @a with a = 1 and push it down to TiKV, it's not an equivalent execution plan.

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