TiFlash 支持的计算下推
本文档介绍 TiFlash 支持的计算下推。
支持下推的算子
TiFlash 支持部分算子的下推,支持的算子如下:
- TableScan:该算子从表中读取数据
- Selection:该算子对数据进行过滤
- HashAgg:该算子基于 Hash Aggregation 算法对数据进行聚合运算
- StreamAgg:该算子基于 Stream Aggregation 算法对数据进行聚合运算。StreamAgg 仅支持不带
GROUP BY
条件的列。 - TopN:该算子对数据求 TopN 运算
- Limit:该算子对数据进行 limit 运算
- Project:该算子对数据进行投影运算
- HashJoin:该算子基于 Hash Join 算法对数据进行连接运算:
- 只有在 MPP 模式下才能被下推
- 支持的 Join 类型包括 Inner Join、Left Join、Semi Join、Anti Semi Join、Left Semi Join、Anti Left Semi Join
- 对于上述类型,既支持带等值条件的连接,也支持不带等值条件的连接(即 Cartesian Join 或者 Null-aware Semi Join);在计算 Cartesian Join 或者 Null-aware Semi Join 时,只会使用 Broadcast 算法,而不会使用 Shuffle Hash Join 算法
- Window:当前支持下推的窗口函数包括
ROW_NUMBER()
、RANK()
、DENSE_RANK()
、LEAD()
、LAG()
、FIRST_VALUE()
和LAST_VALUE()
在 TiDB 中,算子之间会呈现树型组织结构。一个算子能下推到 TiFlash 的前提条件,是该算子的所有子算子都能下推到 TiFlash。因为大部分算子都包含有表达式计算,当且仅当一个算子所包含的所有表达式均支持下推到 TiFlash 时,该算子才有可能下推给 TiFlash。
支持下推的表达式
表达式类型 | 运算 |
---|---|
数学函数 | + , - , / , * , % , >= , <= , = , != , < , > , ROUND() , ABS() , FLOOR(int) , CEIL(int) , CEILING(int) , SQRT() , LOG() , LOG2() , LOG10() , LN() , EXP() , POW() , POWER() , SIGN() , RADIANS() , DEGREES() , CONV() , CRC32() , GREATEST(int/real) , LEAST(int/real) |
逻辑函数和算子 | AND , OR , NOT , CASE WHEN , IF() , IFNULL() , ISNULL() , IN , LIKE , ILIKE , COALESCE , IS |
位运算 | & (bitand), | (bitor), ~ (bitneg), ^ (bitxor) |
字符串函数 | SUBSTR() , CHAR_LENGTH() , REPLACE() , CONCAT() , CONCAT_WS() , LEFT() , RIGHT() , ASCII() , LENGTH() , TRIM() , LTRIM() , RTRIM() , POSITION() , FORMAT() , LOWER() , UCASE() , UPPER() , SUBSTRING_INDEX() , LPAD() , RPAD() , STRCMP() |
正则函数和算子 | REGEXP , REGEXP_LIKE() , REGEXP_INSTR() , REGEXP_SUBSTR() , REGEXP_REPLACE() , RLIKE |
日期函数 | DATE_FORMAT() , TIMESTAMPDIFF() , FROM_UNIXTIME() , UNIX_TIMESTAMP(int) , UNIX_TIMESTAMP(decimal) , STR_TO_DATE(date) , STR_TO_DATE(datetime) , DATEDIFF() , YEAR() , MONTH() , DAY() , EXTRACT(datetime) , DATE() , HOUR() , MICROSECOND() , MINUTE() , SECOND() , SYSDATE() , DATE_ADD/ADDDATE(datetime, int) , DATE_ADD/ADDDATE(string, int/real) , DATE_SUB/SUBDATE(datetime, int) , DATE_SUB/SUBDATE(string, int/real) , QUARTER() , DAYNAME() , DAYOFMONTH() , DAYOFWEEK() , DAYOFYEAR() , LAST_DAY() , MONTHNAME() , TO_SECONDS() , TO_DAYS() , FROM_DAYS() , WEEKOFYEAR() |
JSON 函数 | JSON_LENGTH() , -> , ->> , JSON_EXTRACT() , JSON_ARRAY() , JSON_DEPTH() , JSON_VALID() , JSON_KEYS() , JSON_CONTAINS_PATH() , JSON_UNQUOTE() |
转换函数 | CAST(int AS DOUBLE), CAST(int AS DECIMAL) , CAST(int AS STRING) , CAST(int AS TIME) , CAST(double AS INT) , CAST(double AS DECIMAL) , CAST(double AS STRING) , CAST(double AS TIME) , CAST(string AS INT) , CAST(string AS DOUBLE), CAST(string AS DECIMAL) , CAST(string AS TIME) , CAST(decimal AS INT) , CAST(decimal AS STRING) , CAST(decimal AS TIME) , CAST(decimal AS DOUBLE) , CAST(time AS INT) , CAST(time AS DECIMAL) , CAST(time AS STRING) , CAST(time AS REAL) , CAST(json AS JSON) , CAST(json AS STRING) , CAST(int AS JSON) , CAST(real AS JSON) , CAST(decimal AS JSON) , CAST(string AS JSON) , CAST(time AS JSON) , CAST(duration AS JSON) |
聚合函数 | MIN() , MAX() , SUM() , COUNT() , AVG() , APPROX_COUNT_DISTINCT() , GROUP_CONCAT() |
其他函数 | INET_NTOA() , INET_ATON() , INET6_NTOA() , INET6_ATON() |
下推限制
- 所有包含 Bit、Set 和 Geometry 类型的表达式均不能下推到 TiFlash
DATE_ADD()
、DATE_SUB()
、ADDDATE()
以及SUBDATE()
中的 interval 类型只支持如下几种,如使用了其他类型的 interval,TiFlash 会在运行时报错。- DAY
- WEEK
- MONTH
- YEAR
- HOUR
- MINUTE
- SECOND
如查询遇到不支持的下推计算,则需要依赖 TiDB 完成剩余计算,可能会很大程度影响 TiFlash 加速效果。对于暂不支持的算子/表达式,将会在后续版本中陆续支持。
类似 MAX()
这样的函数在聚合算子中支持下推,但是在窗口函数算子中还不支持下推。
示例
以下通过一些例子对下推算子和表达式到 TiFlash 进行说明。
示例 1:下推算子到 TiFlash 存储
CREATE TABLE t(id INT PRIMARY KEY, a INT);
ALTER TABLE t SET TIFLASH REPLICA 1;
EXPLAIN SELECT * FROM t LIMIT 3;
+------------------------------+---------+--------------+---------------+--------------------------------+
| id | estRows | task | access object | operator info |
+------------------------------+---------+--------------+---------------+--------------------------------+
| Limit_9 | 3.00 | root | | offset:0, count:3 |
| └─TableReader_17 | 3.00 | root | | data:ExchangeSender_16 |
| └─ExchangeSender_16 | 3.00 | mpp[tiflash] | | ExchangeType: PassThrough |
| └─Limit_15 | 3.00 | mpp[tiflash] | | offset:0, count:3 |
| └─TableFullScan_14 | 3.00 | mpp[tiflash] | table:t | keep order:false, stats:pseudo |
+------------------------------+---------+--------------+---------------+--------------------------------+
5 rows in set (0.18 sec)
在该查询中,算子 Limit 被下推到 TiFlash 对数据进行过滤,减少了网络传输数据量,进而减少网络传输开销。具体可查看以上示例中 Limit_15
算子的 task
列,其值为 mpp[tiflash]
,表示该算子被下推到 TiFlash。
示例 2:下推表达式到 TiFlash 存储
CREATE TABLE t(id INT PRIMARY KEY, a INT);
ALTER TABLE t SET TIFLASH REPLICA 1;
INSERT INTO t(id,a) VALUES (1,2),(2,4),(11,2),(12,4),(13,4),(14,7);
EXPLAIN SELECT MAX(id + a) FROM t GROUP BY a;
+------------------------------------+---------+--------------+---------------+---------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+------------------------------------+---------+--------------+---------------+---------------------------------------------------------------------------+
| TableReader_45 | 4.80 | root | | data:ExchangeSender_44 |
| └─ExchangeSender_44 | 4.80 | mpp[tiflash] | | ExchangeType: PassThrough |
| └─Projection_39 | 4.80 | mpp[tiflash] | | Column#3 |
| └─HashAgg_37 | 4.80 | mpp[tiflash] | | group by:Column#9, funcs:max(Column#8)->Column#3 |
| └─Projection_46 | 6.00 | mpp[tiflash] | | plus(test.t.id, test.t.a)->Column#8, test.t.a |
| └─ExchangeReceiver_23 | 6.00 | mpp[tiflash] | | |
| └─ExchangeSender_22 | 6.00 | mpp[tiflash] | | ExchangeType: HashPartition, Hash Cols: [name: test.t.a, collate: binary] |
| └─TableFullScan_21 | 6.00 | mpp[tiflash] | table:t | keep order:false, stats:pseudo |
+------------------------------------+---------+--------------+---------------+---------------------------------------------------------------------------+
8 rows in set (0.18 sec)
在该查询中,表达式 id + a
被下推到 TiFlash,从而能提前进行计算,减少网络传输数据量,进而减少网络传输开销,提升整体计算性能。具体可查看以上示例中 operator
列为 plus(test.t.id, test.t.a)
的行的 task
列,其值为 mpp[tiflash]
,表示该表达式被下推到 TiFlash。
示例 3:下推限制
CREATE TABLE t(id INT PRIMARY KEY, a INT);
ALTER TABLE t SET TIFLASH REPLICA 1;
INSERT INTO t(id,a) VALUES (1,2),(2,4),(11,2),(12,4),(13,4),(14,7);
EXPLAIN SELECT id FROM t WHERE TIME(now()+ a) < '12:00:00';
+-----------------------------+---------+--------------+---------------+--------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+---------+--------------+---------------+--------------------------------------------------------------------------------------------------+
| Projection_4 | 4.80 | root | | test.t.id |
| └─Selection_6 | 4.80 | root | | lt(cast(time(cast(plus(20230110083056, test.t.a), var_string(20))), var_string(10)), "12:00:00") |
| └─TableReader_11 | 6.00 | root | | data:ExchangeSender_10 |
| └─ExchangeSender_10 | 6.00 | mpp[tiflash] | | ExchangeType: PassThrough |
| └─TableFullScan_9 | 6.00 | mpp[tiflash] | table:t | keep order:false, stats:pseudo |
+-----------------------------+---------+--------------+---------------+--------------------------------------------------------------------------------------------------+
5 rows in set, 3 warnings (0.20 sec)
分析执行计划可以发现,该查询在执行时只在 TiFlash 中进行了 TableFullScan,其他的函数计算和过滤均在 root
进行,并未下推至 TiFlash。
执行以下命令,可以查找不能下推的算子和表达式。
SHOW WARNINGS;
+---------+------+------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1105 | Scalar function 'time'(signature: Time, return type: time) is not supported to push down to storage layer now. |
| Warning | 1105 | Scalar function 'cast'(signature: CastDurationAsString, return type: var_string(10)) is not supported to push down to tiflash now. |
| Warning | 1105 | Scalar function 'cast'(signature: CastDurationAsString, return type: var_string(10)) is not supported to push down to tiflash now. |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------+
3 rows in set (0.18 sec)
可以看出,该查询的表达式无法完全下推至 TiFlash,因为 Time
函数和 Cast
函数无法下推至 TiFlash。
示例 4:窗口函数
CREATE TABLE t(id INT PRIMARY KEY, c1 VARCHAR(100));
ALTER TABLE t SET TIFLASH REPLICA 1;
INSERT INTO t VALUES(1,"foo"),(2,"bar"),(3,"bar foo"),(10,"foo"),(20,"bar"),(30,"bar foo");
EXPLAIN SELECT id, ROW_NUMBER() OVER (PARTITION BY id > 10) FROM t;
+----------------------------------+----------+--------------+---------------+---------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+----------------------------------+----------+--------------+---------------+---------------------------------------------------------------------------------------------------------------+
| TableReader_30 | 10000.00 | root | | MppVersion: 1, data:ExchangeSender_29 |
| └─ExchangeSender_29 | 10000.00 | mpp[tiflash] | | ExchangeType: PassThrough |
| └─Projection_7 | 10000.00 | mpp[tiflash] | | test.t.id, Column#5, stream_count: 4 |
| └─Window_28 | 10000.00 | mpp[tiflash] | | row_number()->Column#5 over(partition by Column#4 rows between current row and current row), stream_count: 4 |
| └─Sort_14 | 10000.00 | mpp[tiflash] | | Column#4, stream_count: 4 |
| └─ExchangeReceiver_13 | 10000.00 | mpp[tiflash] | | stream_count: 4 |
| └─ExchangeSender_12 | 10000.00 | mpp[tiflash] | | ExchangeType: HashPartition, Compression: FAST, Hash Cols: [name: Column#4, collate: binary], stream_count: 4 |
| └─Projection_10 | 10000.00 | mpp[tiflash] | | test.t.id, gt(test.t.id, 10)->Column#4 |
| └─TableFullScan_11 | 10000.00 | mpp[tiflash] | table:t | keep order:false, stats:pseudo |
+----------------------------------+----------+--------------+---------------+---------------------------------------------------------------------------------------------------------------+
9 rows in set (0.0073 sec)
可以看到,Window
操作在 task
列中有一个 mpp[tiflash]
的值,表示 ROW_NUMBER() OVER (PARTITION BY id > 10)
操作能够被下推至 TiFlash。
CREATE TABLE t(id INT PRIMARY KEY, c1 VARCHAR(100));
ALTER TABLE t SET TIFLASH REPLICA 1;
INSERT INTO t VALUES(1,"foo"),(2,"bar"),(3,"bar foo"),(10,"foo"),(20,"bar"),(30,"bar foo");
EXPLAIN SELECT id, MAX(id) OVER (PARTITION BY id > 10) FROM t;
+-----------------------------+----------+-----------+---------------+------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+----------+-----------+---------------+------------------------------------------------------------+
| Projection_6 | 10000.00 | root | | test.t1.id, Column#5 |
| └─Shuffle_14 | 10000.00 | root | | execution info: concurrency:5, data sources:[Projection_8] |
| └─Window_7 | 10000.00 | root | | max(test.t1.id)->Column#5 over(partition by Column#4) |
| └─Sort_13 | 10000.00 | root | | Column#4 |
| └─Projection_8 | 10000.00 | root | | test.t1.id, gt(test.t1.id, 10)->Column#4 |
| └─TableReader_10 | 10000.00 | root | | data:TableFullScan_9 |
| └─TableFullScan_9 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+-----------------------------+----------+-----------+---------------+------------------------------------------------------------+
7 rows in set (0.0010 sec)
可以看到,Window
操作在 task
列中有一个 root
的值,表示 MAX(id) OVER (PARTITION BY id > 10)
操作不能被下推至 TiFlash。这是因为,MAX()
只支持作为聚合函数下推,而不支持作为窗口函数下推。