我的书签
添加书签
移除书签用 EXPLAIN 查看聚合查询执行计划
SQL 查询中可能会使用聚合计算，可以通过 EXPLAIN 语句来查看聚合查询的执行计划。本文提供多个示例，以帮助用户理解聚合查询是如何执行的。
SQL 优化器会选择以下任一算子实现数据聚合：
- Hash Aggregation
- Stream Aggregation
为了提高查询效率，数据聚合在 Coprocessor 层和 TiDB 层均会执行。现有示例如下：
CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY auto_increment, pad1 BLOB, pad2 BLOB, pad3 BLOB);INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM dual;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;SELECT SLEEP(1);ANALYZE TABLE t1;
以上示例创建表格 t1 并插入数据后，再执行 SHOW TABLE REGIONS 语句。从以下 SHOW TABLE REGIONS 的执行结果可知，表 t1 被切分为多个 Region：
SHOW TABLE t1 REGIONS;
+-----------+--------------+--------------+-----------+-----------------+-------+------------+---------------+------------+----------------------+------------------+| REGION_ID | START_KEY | END_KEY | LEADER_ID | LEADER_STORE_ID | PEERS | SCATTERING | WRITTEN_BYTES | READ_BYTES | APPROXIMATE_SIZE(MB) | APPROXIMATE_KEYS |+-----------+--------------+--------------+-----------+-----------------+-------+------------+---------------+------------+----------------------+------------------+| 64 | t_64_ | t_64_r_31766 | 65 | 1 | 65 | 0 | 1325 | 102033520 | 98 | 52797 || 66 | t_64_r_31766 | t_64_r_63531 | 67 | 1 | 67 | 0 | 1325 | 72522521 | 104 | 78495 || 68 | t_64_r_63531 | t_64_r_95296 | 69 | 1 | 69 | 0 | 1325 | 0 | 104 | 95433 || 2 | t_64_r_95296 | | 3 | 1 | 3 | 0 | 1501 | 0 | 81 | 63211 |+-----------+--------------+--------------+-----------+-----------------+-------+------------+---------------+------------+----------------------+------------------+4 rows in set (0.00 sec)
使用 EXPLAIN 查看以下聚合语句的执行计划。可以看到 └─StreamAgg_8 算子先执行在 TiKV 内每个 Region 上，然后 TiKV 的每个 Region 会返回一行数据给 TiDB，TiDB 在 StreamAgg_16 算子上对每个 Region 返回的数据进行聚合：
EXPLAIN SELECT COUNT(*) FROM t1;
+----------------------------+-----------+-----------+---------------+---------------------------------+| id | estRows | task | access object | operator info |+----------------------------+-----------+-----------+---------------+---------------------------------+| StreamAgg_16 | 1.00 | root | | funcs:count(Column#7)->Column#5 || └─TableReader_17 | 1.00 | root | | data:StreamAgg_8 || └─StreamAgg_8 | 1.00 | cop[tikv] | | funcs:count(1)->Column#7 || └─TableFullScan_15 | 242020.00 | cop[tikv] | table:t1 | keep order:false |+----------------------------+-----------+-----------+---------------+---------------------------------+4 rows in set (0.00 sec)
同样，通过执行 EXPLAIN ANALYZE 语句可知，actRows 与 SHOW TABLE REGIONS 返回结果中的 Region 数匹配，这是因为执行使用了 TableFullScan 全表扫并且没有二级索引：
EXPLAIN ANALYZE SELECT COUNT(*) FROM t1;
+----------------------------+-----------+---------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------+-----------+------+| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |+----------------------------+-----------+---------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------+-----------+------+| StreamAgg_16 | 1.00 | 1 | root | | time:12.609575ms, loops:2 | funcs:count(Column#7)->Column#5 | 372 Bytes | N/A || └─TableReader_17 | 1.00 | 4 | root | | time:12.605155ms, loops:2, cop_task: {num: 4, max: 12.538245ms, min: 9.256838ms, avg: 10.895114ms, p95: 12.538245ms, max_proc_keys: 31765, p95_proc_keys: 31765, tot_proc: 48ms, rpc_num: 4, rpc_time: 43.530707ms, copr_cache_hit_ratio: 0.00} | data:StreamAgg_8 | 293 Bytes | N/A || └─StreamAgg_8 | 1.00 | 4 | cop[tikv] | | proc max:12ms, min:12ms, p80:12ms, p95:12ms, iters:122, tasks:4 | funcs:count(1)->Column#7 | N/A | N/A || └─TableFullScan_15 | 242020.00 | 121010 | cop[tikv] | table:t1 | proc max:12ms, min:12ms, p80:12ms, p95:12ms, iters:122, tasks:4 | keep order:false | N/A | N/A |+----------------------------+-----------+---------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------+-----------+------+4 rows in set (0.01 sec)
Hash Aggregation
Hash Aggregation 算法在执行聚合时使用 Hash 表存储中间结果。此算法采用多线程并发优化，执行速度快，但与 Stream Aggregation 算法相比会消耗较多内存。
下面是一个使用 Hash Aggregation（即 HashAgg 算子）的例子：
EXPLAIN SELECT /*+ HASH_AGG() */ count(*) FROM t1;
+---------------------------+-----------+-----------+---------------+---------------------------------+| id | estRows | task | access object | operator info |+---------------------------+-----------+-----------+---------------+---------------------------------+| HashAgg_9 | 1.00 | root | | funcs:count(Column#6)->Column#5 || └─TableReader_10 | 1.00 | root | | data:HashAgg_5 || └─HashAgg_5 | 1.00 | cop[tikv] | | funcs:count(1)->Column#6 || └─TableFullScan_8 | 242020.00 | cop[tikv] | table:t1 | keep order:false |+---------------------------+-----------+-----------+---------------+---------------------------------+4 rows in set (0.00 sec)
operator info 列显示，用于聚合数据的 Hash 函数为 funcs:count(1)->Column#6。
Stream Aggregation
Stream Aggregation 算法通常会比 Hash Aggregation 算法占用更少的内存。但是此算法要求数据按顺序发送，以便对依次到达的值实现流式数据聚合。
下面是一个使用 Stream Aggregation 的例子：
CREATE TABLE t2 (id INT NOT NULL PRIMARY KEY, col1 INT NOT NULL);INSERT INTO t2 VALUES (1, 9),(2, 3),(3,1),(4,8),(6,3);EXPLAIN SELECT /*+ STREAM_AGG() */ col1, count(*) FROM t2 GROUP BY col1;
Query OK, 0 rows affected (0.11 sec)Query OK, 5 rows affected (0.01 sec)Records: 5 Duplicates: 0 Warnings: 0+------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------+| Projection_4 | 8000.00 | root | | test.t2.col1, Column#3 || └─StreamAgg_8 | 8000.00 | root | | group by:test.t2.col1, funcs:count(1)->Column#3, funcs:firstrow(test.t2.col1)->test.t2.col1 || └─Sort_13 | 10000.00 | root | | test.t2.col1 || └─TableReader_12 | 10000.00 | root | | data:TableFullScan_11 || └─TableFullScan_11 | 10000.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |+------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------+5 rows in set (0.00 sec)
以上示例中，可以在 col1 上添加索引来消除 └─Sort_13 算子。添加索引后，TiDB 就可以按顺序读取数据并消除 └─Sort_13 算子。
ALTER TABLE t2 ADD INDEX (col1);EXPLAIN SELECT /*+ STREAM_AGG() */ col1, count(*) FROM t2 GROUP BY col1;
Query OK, 0 rows affected (0.28 sec)+------------------------------+---------+-----------+----------------------------+----------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+------------------------------+---------+-----------+----------------------------+----------------------------------------------------------------------------------------------------+| Projection_4 | 4.00 | root | | test.t2.col1, Column#3 || └─StreamAgg_14 | 4.00 | root | | group by:test.t2.col1, funcs:count(Column#4)->Column#3, funcs:firstrow(test.t2.col1)->test.t2.col1 || └─IndexReader_15 | 4.00 | root | | index:StreamAgg_8 || └─StreamAgg_8 | 4.00 | cop[tikv] | | group by:test.t2.col1, funcs:count(1)->Column#4 || └─IndexFullScan_13 | 5.00 | cop[tikv] | table:t2, index:col1(col1) | keep order:true, stats:pseudo |+------------------------------+---------+-----------+----------------------------+----------------------------------------------------------------------------------------------------+5 rows in set (0.00 sec)
多维度数据聚合 ROLLUP
自 v7.4.0 起，TiDB 的 GROUP BY 子句支持 WITH ROLLUP 修饰符。
你可以在 GROUP BY 子句中指定一个或多个列，形成一个分组列表，然后添加 WITH ROLLUP 修饰符。TiDB 将会按照分组列表中的列进行多维度的递减分组，并在输出中为你提供各个分组数据的汇总结果。
注意
TiDB 暂不支持 Cube 语法; TiDB 目前仅在 MPP 模式下支持为 WITH ROLLUP 语法生成有效的执行计划。
explain SELECT year, month, grouping(year), grouping(month), SUM(profit) AS profit FROM bank GROUP BY year, month WITH ROLLUP;+----------------------------------------+---------+--------------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+----------------------------------------+---------+--------------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| TableReader_44 | 2.40 | root | | MppVersion: 2, data:ExchangeSender_43 || └─ExchangeSender_43 | 2.40 | mpp[tiflash] | | ExchangeType: PassThrough || └─Projection_8 | 2.40 | mpp[tiflash] | | Column#6->Column#12, Column#7->Column#13, grouping(gid)->Column#14, grouping(gid)->Column#15, Column#9->Column#16 || └─Projection_38 | 2.40 | mpp[tiflash] | | Column#9, Column#6, Column#7, gid || └─HashAgg_36 | 2.40 | mpp[tiflash] | | group by:Column#6, Column#7, gid, funcs:sum(test.bank.profit)->Column#9, funcs:firstrow(Column#6)->Column#6, funcs:firstrow(Column#7)->Column#7, funcs:firstrow(gid)->gid, stream_count: 8 || └─ExchangeReceiver_22 | 3.00 | mpp[tiflash] | | stream_count: 8 || └─ExchangeSender_21 | 3.00 | mpp[tiflash] | | ExchangeType: HashPartition, Compression: FAST, Hash Cols: [name: Column#6, collate: binary], [name: Column#7, collate: utf8mb4_bin], [name: gid, collate: binary], stream_count: 8 || └─Expand_20 | 3.00 | mpp[tiflash] | | level-projection:[test.bank.profit, <nil>->Column#6, <nil>->Column#7, 0->gid],[test.bank.profit, Column#6, <nil>->Column#7, 1->gid],[test.bank.profit, Column#6, Column#7, 3->gid]; schema: [test.bank.profit,Column#6,Column#7,gid] || └─Projection_16 | 3.00 | mpp[tiflash] | | test.bank.profit, test.bank.year->Column#6, test.bank.month->Column#7 || └─TableFullScan_17 | 3.00 | mpp[tiflash] | table:bank | keep order:false, stats:pseudo |+----------------------------------------+---------+--------------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+10 rows in set (0.05 sec)
该语句的 SQL 聚合可以按照 GROUP BY year, month WITH ROLLUP 语法在 {year, month}、{year}、{} 这 3 个分组中分别计算并连接结果。
更多信息，请参考 GROUP BY 修饰符。
