我在一个基本上收集日志信息的表上经常执行以下两个查询。两者都从大量行中选择不同的值,但其中的不同值少于 10 个。
我分析了该页面完成的两个“不同”查询:
marchena=> explain select distinct auditrecor0_.bundle_id as col_0_0_ from audit_records auditrecor0_;
QUERY PLAN
----------------------------------------------------------------------------------------------
HashAggregate (cost=1070734.05..1070734.11 rows=6 width=21)
-> Seq Scan on audit_records auditrecor0_ (cost=0.00..1023050.24 rows=19073524 width=21)
(2 rows)
marchena=> explain select distinct auditrecor0_.server_name as col_0_0_ from audit_records auditrecor0_;
QUERY PLAN
----------------------------------------------------------------------------------------------
HashAggregate (cost=1070735.34..1070735.39 rows=5 width=13)
-> Seq Scan on audit_records auditrecor0_ (cost=0.00..1023051.47 rows=19073547 width=13)
(2 rows)
两者都对列进行顺序扫描。但是,如果我关闭enable_seqscan(尽管名称如此,这只会禁用对具有索引的列进行序列扫描),则查询将使用索引,但速度会更慢:
marchena=> set enable_seqscan = off;
SET
marchena=> explain select distinct auditrecor0_.bundle_id as col_0_0_ from audit_records auditrecor0_;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
Unique (cost=0.00..19613740.62 rows=6 width=21)
-> Index Scan using audit_bundle_idx on audit_records auditrecor0_ (cost=0.00..19566056.69 rows=19073570 width=21)
(2 rows)
marchena=> explain select distinct auditrecor0_.server_name as col_0_0_ from audit_records auditrecor0_;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
Unique (cost=0.00..45851449.96 rows=5 width=13)
-> Index Scan using audit_server_idx on audit_records auditrecor0_ (cost=0.00..45803766.04 rows=19073570 width=13)
(2 rows)
bundle_id 和 server_name 列都有 btree 索引,我是否应该使用不同类型的索引来快速选择不同的值?
BEGIN;
CREATE TABLE dist ( x INTEGER NOT NULL );
INSERT INTO dist SELECT random()*50 FROM generate_series( 1, 5000000 );
COMMIT;
CREATE INDEX dist_x ON dist(x);
VACUUM ANALYZE dist;
EXPLAIN ANALYZE SELECT DISTINCT x FROM dist;
HashAggregate (cost=84624.00..84624.51 rows=51 width=4) (actual time=1840.141..1840.153 rows=51 loops=1)
-> Seq Scan on dist (cost=0.00..72124.00 rows=5000000 width=4) (actual time=0.003..573.819 rows=5000000 loops=1)
Total runtime: 1848.060 ms
PG(还)不能使用不同的索引(跳过相同的值),但你可以这样做:
CREATE OR REPLACE FUNCTION distinct_skip_foo()
RETURNS SETOF INTEGER
LANGUAGE plpgsql STABLE
AS $$
DECLARE
_x INTEGER;
BEGIN
_x := min(x) FROM dist;
WHILE _x IS NOT NULL LOOP
RETURN NEXT _x;
_x := min(x) FROM dist WHERE x > _x;
END LOOP;
END;
$$ ;
EXPLAIN ANALYZE SELECT * FROM distinct_skip_foo();
Function Scan on distinct_skip_foo (cost=0.00..260.00 rows=1000 width=4) (actual time=1.629..1.635 rows=51 loops=1)
Total runtime: 1.652 ms
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