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15623c7f29
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)
322 lines
8.2 KiB
Text
322 lines
8.2 KiB
Text
#
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# MDEV-30660 COUNT DISTINCT seems unnecessarily slow when run on a PK
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#
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set @save_optimizer_trace = @@optimizer_trace;
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SET optimizer_trace='enabled=on';
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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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# Optimization is applied (aggregator=simple):
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SELECT COUNT(DISTINCT a) FROM t1;
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COUNT(DISTINCT a)
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3
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t1.a)",
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"aggregator_type": "simple"
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}
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]
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SELECT AVG(DISTINCT a), SUM(DISTINCT b) FROM t1;
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AVG(DISTINCT a) SUM(DISTINCT b)
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2.0000 1
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "avg(distinct t1.a)",
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"aggregator_type": "simple"
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},
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{
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"function": "sum(distinct t1.b)",
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"aggregator_type": "distinct"
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}
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]
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# 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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COUNT(DISTINCT b, a)
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3
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t1.b,t1.a)",
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"aggregator_type": "simple"
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}
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]
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SELECT COUNT(DISTINCT a, a + b) FROM t1;
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COUNT(DISTINCT a, a + b)
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3
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t1.a,t1.a + t1.b)",
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"aggregator_type": "simple"
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}
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]
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SELECT SUM(DISTINCT a), AVG(DISTINCT a), COUNT(DISTINCT a) FROM t1 WHERE a > 1;
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SUM(DISTINCT a) AVG(DISTINCT a) COUNT(DISTINCT a)
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5 2.5000 2
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "sum(distinct t1.a)",
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"aggregator_type": "simple"
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},
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{
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"function": "avg(distinct t1.a)",
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"aggregator_type": "simple"
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},
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{
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"function": "count(distinct t1.a)",
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"aggregator_type": "simple"
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}
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]
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# Optimization is not applied 'cause function argument is not a field
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# (aggregator=distinct):
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SELECT SUM(DISTINCT a + b) FROM t1;
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SUM(DISTINCT a + b)
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9
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "sum(distinct t1.a + t1.b)",
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"aggregator_type": "distinct"
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}
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]
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SELECT COUNT(DISTINCT b) FROM t1;
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COUNT(DISTINCT b)
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1
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t1.b)",
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"aggregator_type": "distinct"
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}
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]
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SELECT AVG(DISTINCT b / a) FROM t1;
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AVG(DISTINCT b / a)
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0.61110000
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "avg(distinct t1.b / t1.a)",
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"aggregator_type": "distinct"
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}
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]
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EXPLAIN SELECT COUNT(DISTINCT (SELECT a)) FROM t1;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 index NULL PRIMARY 4 NULL 3 Using index
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2 DEPENDENT SUBQUERY NULL NULL NULL NULL NULL NULL NULL No tables used
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct (/* select#2 */ select t1.a))",
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"aggregator_type": "distinct"
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}
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]
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CREATE TABLE t2 (a INT);
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INSERT INTO t2 VALUES (1), (2);
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# Optimization is not applied 'cause there is more than one table
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SELECT COUNT(DISTINCT t1.a) FROM t1, t2;
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COUNT(DISTINCT t1.a)
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3
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t1.a)",
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"aggregator_type": "distinct"
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}
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]
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SELECT AVG(DISTINCT t1.a) FROM t1, t2;
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AVG(DISTINCT t1.a)
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2.0000
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "avg(distinct t1.a)",
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"aggregator_type": "distinct"
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}
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]
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# Const tables, optimization is applied
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SELECT COUNT(DISTINCT a) FROM t1, (SELECT 1) AS t2;
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COUNT(DISTINCT a)
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3
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t1.a)",
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"aggregator_type": "simple"
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}
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]
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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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AVG(DISTINCT t1.a)
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2.0000
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "avg(distinct t1.a)",
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"aggregator_type": "simple"
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}
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]
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SELECT COUNT(DISTINCT a) FROM t1, (SELECT 1 UNION SELECT 2) AS t2;
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COUNT(DISTINCT a)
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3
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t1.a)",
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"aggregator_type": "distinct"
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}
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]
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# 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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# Optimization is applied:
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SELECT COUNT(DISTINCT a, b) FROM t3;
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COUNT(DISTINCT a, b)
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7
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t3.a,t3.b)",
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"aggregator_type": "simple"
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}
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]
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SELECT COUNT(DISTINCT b, a) FROM t3;
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COUNT(DISTINCT b, a)
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7
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t3.b,t3.a)",
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"aggregator_type": "simple"
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}
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]
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SELECT COUNT(DISTINCT b, a) FROM t3 WHERE a < 3;
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COUNT(DISTINCT b, a)
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5
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t3.b,t3.a)",
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"aggregator_type": "simple"
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}
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]
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# Optimization is applied to one of the functions:
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SELECT COUNT(DISTINCT b), SUM(DISTINCT a), SUM(DISTINCT a + b) FROM t3 GROUP BY a;
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COUNT(DISTINCT b) SUM(DISTINCT a) SUM(DISTINCT a + b)
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3 1 9
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2 2 7
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2 3 9
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t3.b)",
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"aggregator_type": "simple"
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},
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{
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"function": "sum(distinct t3.a)",
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"aggregator_type": "distinct"
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},
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{
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"function": "sum(distinct t3.a + t3.b)",
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"aggregator_type": "distinct"
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}
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]
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# 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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COUNT(DISTINCT b)
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1
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2
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3
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1
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t3.b)",
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"aggregator_type": "distinct"
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}
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]
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# Test merged view
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CREATE VIEW v1 AS SELECT * FROM t1;
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# Optimization is applied
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SELECT COUNT(DISTINCT a, b) FROM v1;
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COUNT(DISTINCT a, b)
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3
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "count(distinct t1.a,t1.b)",
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"aggregator_type": "simple"
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}
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]
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# GROUP_CONCAT implements non-standard distinct aggregator
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SELECT GROUP_CONCAT(b) FROM t1;
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GROUP_CONCAT(b)
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1,1,1
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "group_concat(t1.b separator ',')",
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"aggregator_type": "simple"
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}
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]
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SELECT GROUP_CONCAT(DISTINCT b) FROM t1;
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GROUP_CONCAT(DISTINCT b)
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1
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SELECT JSON_DETAILED(JSON_EXTRACT(trace, '$**.prepare_sum_aggregators')) AS JS
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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JS
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[
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{
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"function": "group_concat(distinct t1.b separator ',')",
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"aggregator_type": "distinct"
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}
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]
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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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#
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# end of 10.5 tests
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#
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