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When executing a statement of the form
SELECT AGGR_FN(DISTINCT c1, c2,..,cn) FROM t1,
where AGGR_FN is an aggregate function such as COUNT(), AVG() or SUM(),
and a unique index exists on table t1 covering some or all of the
columns (c1, c2,..,cn), the retrieved values are inherently unique.
Consequently, the need for de-duplication imposed by the DISTINCT
clause can be eliminated, leading to optimization of aggregation
operations.
This optimization applies under the following conditions:
- only one table involved in the join (not counting const tables)
- some arguments of the aggregate function are fields
(not functions/subqueries)
This optimization extends to queries of the form
SELECT AGGR_FN(c1, c2,..,cn) GROUP BY cx,..cy
when a unique index covers some or all of the columns
(c1, c2,..cn, cx,..cy)
109 lines
2.9 KiB
Text
109 lines
2.9 KiB
Text
# Embedded doesn't have optimizer trace:
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--source include/not_embedded.inc
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--source include/have_sequence.inc
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--echo #
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--echo # MDEV-30660 COUNT DISTINCT seems unnecessarily slow when run on a PK
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--echo #
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set @save_optimizer_trace = @@optimizer_trace;
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SET optimizer_trace='enabled=on';
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let $trace=
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '\$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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CREATE TABLE t1 (a INT NOT NULL PRIMARY KEY, b INT NOT NULL);
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INSERT INTO t1 VALUES (1,1), (2,1), (3,1);
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--echo # Optimization is applied (aggregator=simple):
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SELECT COUNT(DISTINCT a) FROM t1;
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eval $trace;
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SELECT AVG(DISTINCT a), SUM(DISTINCT b) FROM t1;
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eval $trace;
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--echo # Only `a` is unique but it's enough to eliminate DISTINCT:
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SELECT COUNT(DISTINCT b, a) FROM t1;
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eval $trace;
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SELECT COUNT(DISTINCT a, a + b) FROM t1;
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eval $trace;
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SELECT SUM(DISTINCT a), AVG(DISTINCT a), COUNT(DISTINCT a) FROM t1 WHERE a > 1;
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eval $trace;
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--echo # Optimization is not applied 'cause function argument is not a field
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--echo # (aggregator=distinct):
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SELECT SUM(DISTINCT a + b) FROM t1;
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eval $trace;
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SELECT COUNT(DISTINCT b) FROM t1;
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eval $trace;
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SELECT AVG(DISTINCT b / a) FROM t1;
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eval $trace;
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EXPLAIN SELECT COUNT(DISTINCT (SELECT a)) FROM t1;
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eval $trace;
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CREATE TABLE t2 (a INT);
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INSERT INTO t2 VALUES (1), (2);
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--echo # Optimization is not applied 'cause there is more than one table
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SELECT COUNT(DISTINCT t1.a) FROM t1, t2;
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eval $trace;
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SELECT AVG(DISTINCT t1.a) FROM t1, t2;
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eval $trace;
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--echo # Const tables, optimization is applied
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SELECT COUNT(DISTINCT a) FROM t1, (SELECT 1) AS t2;
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eval $trace;
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SELECT AVG(DISTINCT t1.a) FROM (SELECT 1 AS a) AS t2, t1, (SELECT 2 AS a) AS t3;
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eval $trace;
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SELECT COUNT(DISTINCT a) FROM t1, (SELECT 1 UNION SELECT 2) AS t2;
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eval $trace;
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--echo # Unique index on two columns
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CREATE TABLE t3 (a INT NOT NULL, b INT NOT NULL);
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INSERT INTO t3 VALUES (1,1), (1,2), (1,3), (2,1), (2,2), (3,1), (3,2);
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CREATE UNIQUE INDEX t3_a_b ON t3 (a, b);
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--echo # Optimization is applied:
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SELECT COUNT(DISTINCT a, b) FROM t3;
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eval $trace;
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SELECT COUNT(DISTINCT b, a) FROM t3;
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eval $trace;
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SELECT COUNT(DISTINCT b, a) FROM t3 WHERE a < 3;
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eval $trace;
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--echo # Optimization is applied to one of the functions:
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SELECT COUNT(DISTINCT b), SUM(DISTINCT a), SUM(DISTINCT a + b) FROM t3 GROUP BY a;
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eval $trace;
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--echo # Can't apply optimization 'cause GROUP BY argument is not a field:
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SELECT COUNT(DISTINCT b) FROM t3 GROUP BY a+b;
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eval $trace;
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--echo # Test merged view
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CREATE VIEW v1 AS SELECT * FROM t1;
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--echo # Optimization is applied
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SELECT COUNT(DISTINCT a, b) FROM v1;
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eval $trace;
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--echo # GROUP_CONCAT implements non-standard distinct aggregator
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SELECT GROUP_CONCAT(b) FROM t1;
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eval $trace;
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SELECT GROUP_CONCAT(DISTINCT b) FROM t1;
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eval $trace;
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DROP TABLE t1, t2, t3;
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DROP VIEW v1;
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SET optimizer_trace = @save_optimizer_trace;
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--echo #
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--echo # end of 10.5 tests
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--echo #
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