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這篇文章主要介紹“PostgreSQL新特性分析”，在日常操作中，相信很多人在PostgreSQL新特性分析問題上存在疑惑，小編查閱了各式資料，整理出簡單好用的操作方法，希望對大家解答”PostgreSQL新特性分析”的疑惑有所幫助！接下來，請跟著小編一起來學習吧！
在PG 8.4 ~ PG 11,PG會把WITH中的查詢視為”optimization fence”(優化圍欄,與WITH外的查詢隔離,獨立優化),也就意味著謂詞下推等優化手段無法應用到WITH子句中,考慮到CTE在大多數情況下是為了增強可讀性而存在,因此在PG 12中,滿足以下三個條件的,優化器將不會對CTE”視而不見”而是執行”積極的”優化.
A.遞歸查詢
B.沒有任何副作用(side effect)
C.僅在查詢的后續部分引用一次
謂詞下推
測試腳本:
drop table if exists t_w1; drop table if exists t_w2; drop table if exists t_w3; create table t_w1(id int ,c1 varchar(20)); create table t_w2(id int ,c1 varchar(20)); create table t_w3(id int ,c1 varchar(20)); insert into t_w1 select x,x||'' from generate_series(1,10000) as x; insert into t_w2 select x/2,(x/2)||'' from generate_series(1,10000) as x; insert into t_w3 select x,x||'' from generate_series(1,10000) as x;
查詢語句:
WITH t1 AS ( SELECT * FROM t_w1 WHERE t_w1.id % 4 = 0 ) SELECT * FROM t1 JOIN t_w2 as t2 ON t2.id = t1.id AND t1.id < 100;
在PG 11中,其執行計劃如下:
version -------------------------------------------------------------------------------------------- PostgreSQL 11.2 on x86_XX-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-16), XX-bit (1 row) testdb=# explain analyze WITH t1 AS ( SELECT * FROM t_w1 WHERE t_w1.id % 4 = 0 ) testdb-# SELECT * FROM t1 testdb-# JOIN t_w2 as t2 testdb-# ON t2.id = t1.id testdb-# AND t1.id < 100; QUERY PLAN -------------------------------------------------------------------------------------------- Hash Join (cost=205.34..396.18 rows=34 width=70) (actual time=8.576..11.187 rows=48 loops=1) Hash Cond: (t2.id = t1.id) CTE t1 -> Seq Scan on t_w1 (cost=0.00..204.00 rows=50 width=8) (actual time=0.029..6.074 rows=2500 loops=1) Filter: ((id % 4) = 0) Rows Removed by Filter: 7500 -> Seq Scan on t_w2 t2 (cost=0.00..153.00 rows=10000 width=8) (actual time=0.030..1.166 rows=10000 loops=1) -> Hash (cost=1.12..1.12 rows=17 width=62) (actual time=8.536..8.536 rows=24 loops=1) Buckets: 1024 Batches: 1 Memory Usage: 9kB -> CTE Scan on t1 (cost=0.00..1.12 rows=17 width=62) (actual time=0.033..8.521 rows=24 loops=1) Filter: (id < 100) Rows Removed by Filter: 2476 Planning Time: 1.913 ms Execution Time: 11.357 ms (14 rows)
在PG 12中,其執行計劃如下:
testdb=# select version(); version -------------------------------------------------------------------------------------------- PostgreSQL 12beta1 on x86_XX-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-16), XX-bit (1 row) testdb=# explain analyze WITH t1 AS ( SELECT * FROM t_w1 WHERE t_w1.id % 4 = 0 ) testdb-# SELECT * FROM t1 testdb-# JOIN t_w2 as t2 testdb-# ON t2.id = t1.id testdb-# AND t1.id < 100; QUERY PLAN -------------------------------------------------------------------------------------------- Hash Join (cost=229.01..419.52 rows=1 width=16) (actual time=6.974..17.156 rows=48 loops=1) Hash Cond: (t2.id = t_w1.id) -> Seq Scan on t_w2 t2 (cost=0.00..153.00 rows=10000 width=8) (actual time=0.076..5.205 rows=10000 loops=1) -> Hash (cost=229.00..229.00 rows=1 width=8) (actual time=6.882..6.882 rows=24 loops=1) Buckets: 1024 Batches: 1 Memory Usage: 9kB -> Seq Scan on t_w1 (cost=0.00..229.00 rows=1 width=8) (actual time=0.077..6.842 rows=24 loops=1) Filter: ((id < 100) AND ((id % 4) = 0)) Rows Removed by Filter: 9976 Planning Time: 1.677 ms Execution Time: 17.244 ms (10 rows)
可以看到,在PG 11中,謂詞(id < 100)不會下推CTE中,但在PG 12中,優化器則把謂詞下推到CTE中(Filter: ((id < 100) AND ((id % 4) = 0))).
New Option
如果希望12的優化器行為與先前的一樣,則加入Option : MATERIALIZED.
testdb=# explain analyze WITH t1 AS MATERIALIZED( SELECT * FROM t_w1 WHERE t_w1.id % 4 = 0 ) SELECT * FROM t1 JOIN t_w2 as t2 ON t2.id = t1.id AND t1.id < 100; QUERY PLAN ------------------------------------------------------------------------------------------- Hash Join (cost=205.34..396.18 rows=34 width=70) (actual time=30.705..48.549 rows=48 loops=1) Hash Cond: (t2.id = t1.id) CTE t1 -> Seq Scan on t_w1 (cost=0.00..204.00 rows=50 width=8) (actual time=0.152..21.274 rows=2500 loops=1) Filter: ((id % 4) = 0) Rows Removed by Filter: 7500 -> Seq Scan on t_w2 t2 (cost=0.00..153.00 rows=10000 width=8) (actual time=0.154..8.582 rows=10000 loops=1) -> Hash (cost=1.12..1.12 rows=17 width=62) (actual time=30.502..30.502 rows=24 loops=1) Buckets: 1024 Batches: 1 Memory Usage: 9kB -> CTE Scan on t1 (cost=0.00..1.12 rows=17 width=62) (actual time=0.168..30.445 rows=24 loops=1) Filter: (id < 100) Rows Removed by Filter: 2476 Planning Time: 7.673 ms Execution Time: 49.284 ms (14 rows)
如果希望優化器把盡可能的把CTE視為內聯查詢進行優化,則指定NOT MATERIALIZED Option:
下面的查詢,CTE被引用多次,優化器默認會進行MATERIALIZED,通過指定NOT MATERIALIZED則強制為內聯查詢.
testdb=# explain analyze WITH t1 AS ( SELECT * FROM t_w1 WHERE t_w1.id % 4 = 0 ) testdb-# SELECT * FROM t1 testdb-# JOIN t_w2 as t2 testdb-# ON t2.id = t1.id testdb-# UNION ALL testdb-# select t1.*,NULL,NULL from t1 where t1.id % 3 = 0; QUERY PLAN ----------------------------------------------------------------------------------------------------------------------- Append (cost=205.62..399.89 rows=101 width=70) (actual time=11.663..27.725 rows=3332 loops=1) CTE t1 -> Seq Scan on t_w1 (cost=0.00..204.00 rows=50 width=8) (actual time=0.032..7.300 rows=2500 loops=1) Filter: ((id % 4) = 0) Rows Removed by Filter: 7500 -> Hash Join (cost=1.62..193.12 rows=100 width=70) (actual time=11.662..24.094 rows=2499 loops=1) Hash Cond: (t2.id = t1.id) -> Seq Scan on t_w2 t2 (cost=0.00..153.00 rows=10000 width=8) (actual time=0.033..4.412 rows=10000 loops=1) -> Hash (cost=1.00..1.00 rows=50 width=62) (actual time=11.611..11.612 rows=2500 loops=1) Buckets: 4096 (originally 1024) Batches: 1 (originally 1) Memory Usage: 132kB -> CTE Scan on t1 (cost=0.00..1.00 rows=50 width=62) (actual time=0.035..9.916 rows=2500 loops=1) -> CTE Scan on t1 t1_1 (cost=0.00..1.25 rows=1 width=98) (actual time=0.008..2.824 rows=833 loops=1) Filter: ((id % 3) = 0) Rows Removed by Filter: 1667 Planning Time: 2.358 ms Execution Time: 28.746 ms (16 rows)
使用NOT MATERIALIZED選項
testdb=# explain analyze WITH t1 AS NOT MATERIALIZED( SELECT * FROM t_w1 WHERE t_w1.id % 4 = 0 ) SELECT * FROM t1 JOIN t_w2 as t2 ON t2.id = t1.id UNION ALL select t1.*,NULL,NULL from t1 where t1.id % 3 = 0; QUERY PLAN ------------------------------------------------------------------------------------------- Append (cost=204.62..650.39 rows=51 width=17) (actual time=27.894..57.453 rows=3332 loops=1) -> Hash Join (cost=204.62..395.62 rows=50 width=16) (actual time=27.892..48.911 rows=2499 loops=1) Hash Cond: (t2.id = t_w1.id) -> Seq Scan on t_w2 t2 (cost=0.00..153.00 rows=10000 width=8) (actual time=0.149..7.606 rows=10000 loops=1) -> Hash (cost=204.00..204.00 rows=50 width=8) (actual time=27.699..27.699 rows=2500 loops=1) Buckets: 4096 (originally 1024) Batches: 1 (originally 1) Memory Usage: 132kB -> Seq Scan on t_w1 (cost=0.00..204.00 rows=50 width=8) (actual time=0.151..22.446 rows=2500 loops=1) Filter: ((id % 4) = 0) Rows Removed by Filter: 7500 -> Seq Scan on t_w1 t_w1_1 (cost=0.00..254.00 rows=1 width=44) (actual time=0.038..7.400 rows=833 loops=1) Filter: (((id % 4) = 0) AND ((id % 3) = 0)) Rows Removed by Filter: 9167 Planning Time: 12.357 ms Execution Time: 58.490 ms (14 rows)
到此，關于“PostgreSQL新特性分析”的學習就結束了，希望能夠解決大家的疑惑。理論與實踐的搭配能更好的幫助大家學習，快去試試吧！若想繼續學習更多相關知識，請繼續關注億速云網站，小編會繼續努力為大家帶來更多實用的文章！
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