PostgreSQL LIKE 查詢效率提升實驗<轉>

一、未做索引的查詢效率

作為對比，先對未索引的查詢做測試

EXPLAIN ANALYZE select * from gallery_map where author = '曹志耘'; QUERY PLAN ----------------------------------------------------------------------------------------------------------------- Seq Scan on gallery_map (cost=0.00..7002.32 rows=1025 width=621) (actual time=0.011..39.753 rows=1031 loops=1) Filter: ((author)::text = '曹志耘'::text) Rows Removed by Filter: 71315 Planning time: 0.194 ms Execution time: 39.879 ms (5 rows) Time: 40.599 ms 

EXPLAIN ANALYZE select * from gallery_map where author like '曹志耘'; QUERY PLAN ----------------------------------------------------------------------------------------------------------------- Seq Scan on gallery_map (cost=0.00..7002.32 rows=1025 width=621) (actual time=0.017..41.513 rows=1031 loops=1) Filter: ((author)::text ~~ '曹志耘'::text) Rows Removed by Filter: 71315 Planning time: 0.188 ms Execution time: 41.669 ms (5 rows) Time: 42.457 ms 

EXPLAIN ANALYZE select * from gallery_map where author like '曹志耘%'; QUERY PLAN ----------------------------------------------------------------------------------------------------------------- Seq Scan on gallery_map (cost=0.00..7002.32 rows=1028 width=621) (actual time=0.017..41.492 rows=1031 loops=1) Filter: ((author)::text ~~ '曹志耘%'::text) Rows Removed by Filter: 71315 Planning time: 0.307 ms Execution time: 41.633 ms (5 rows) Time: 42.676 ms

很顯然都會做全表掃描

二、創建btree索引

PostgreSQL默認索引是btree

CREATE INDEX ix_gallery_map_author ON gallery_map (author); 

EXPLAIN ANALYZE select * from gallery_map where author = '曹志耘'; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on gallery_map (cost=36.36..2715.37 rows=1025 width=621) (actual time=0.457..1.312 rows=1031 loops=1) Recheck Cond: ((author)::text = '曹志耘'::text) Heap Blocks: exact=438 -> Bitmap Index Scan on ix_gallery_map_author (cost=0.00..36.10 rows=1025 width=0) (actual time=0.358..0.358 rows=1031 loops=1) Index Cond: ((author)::text = '曹志耘'::text) Planning time: 0.416 ms Execution time: 1.422 ms (7 rows) Time: 2.462 ms 

EXPLAIN ANALYZE select * from gallery_map where author like '曹志耘'; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on gallery_map (cost=36.36..2715.37 rows=1025 width=621) (actual time=0.752..2.119 rows=1031 loops=1) Filter: ((author)::text ~~ '曹志耘'::text) Heap Blocks: exact=438 -> Bitmap Index Scan on ix_gallery_map_author (cost=0.00..36.10 rows=1025 width=0) (actual time=0.560..0.560 rows=1031 loops=1) Index Cond: ((author)::text = '曹志耘'::text) Planning time: 0.270 ms Execution time: 2.295 ms (7 rows) Time: 3.444 ms 

EXPLAIN ANALYZE select * from gallery_map where author like '曹志耘%'; QUERY PLAN ----------------------------------------------------------------------------------------------------------------- Seq Scan on gallery_map (cost=0.00..7002.32 rows=1028 width=621) (actual time=0.015..41.389 rows=1031 loops=1) Filter: ((author)::text ~~ '曹志耘%'::text) Rows Removed by Filter: 71315 Planning time: 0.260 ms Execution time: 41.518 ms (5 rows) Time: 42.430 ms

EXPLAIN ANALYZE select * from gallery_map where author like '%研究室'; QUERY PLAN ----------------------------------------------------------------------------------------------------------------- Seq Scan on gallery_map (cost=0.00..7002.32 rows=2282 width=621) (actual time=0.064..52.824 rows=2152 loops=1) Filter: ((author)::text ~~ '%研究室'::text) Rows Removed by Filter: 70194 Planning time: 0.254 ms Execution time: 53.064 ms (5 rows) Time: 53.954 ms 

可以看到，等於、like的全匹配是用到索引的，like的模糊查詢還是全表掃描

三、創建gin索引

CREATE EXTENSION pg_trgm; CREATE INDEX ix_gallery_map_author ON gallery_map USING gin (author gin_trgm_ops);

EXPLAIN ANALYZE select * from gallery_map where author like '曹%'; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on gallery_map (cost=19.96..2705.69 rows=1028 width=621) (actual time=0.419..1.771 rows=1031 loops=1) Recheck Cond: ((author)::text ~~ '曹%'::text) Heap Blocks: exact=438 -> Bitmap Index Scan on ix_gallery_map_author (cost=0.00..19.71 rows=1028 width=0) (actual time=0.312..0.312 rows=1031 loops=1) Index Cond: ((author)::text ~~ '曹%'::text) Planning time: 0.358 ms Execution time: 1.916 ms (7 rows) Time: 2.843 ms

EXPLAIN ANALYZE select * from gallery_map where author like '%耘%'; QUERY PLAN ----------------------------------------------------------------------------------------------------------------- Seq Scan on gallery_map (cost=0.00..7002.32 rows=1028 width=621) (actual time=0.015..51.641 rows=1031 loops=1) Filter: ((author)::text ~~ '%耘%'::text) Rows Removed by Filter: 71315 Planning time: 0.268 ms Execution time: 51.957 ms (5 rows) Time: 52.899 ms

EXPLAIN ANALYZE select * from gallery_map where author like '%研究室%'; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on gallery_map (cost=31.83..4788.42 rows=2559 width=621) (actual time=0.914..4.195 rows=2402 loops=1) Recheck Cond: ((author)::text ~~ '%研究室%'::text) Heap Blocks: exact=868 -> Bitmap Index Scan on ix_gallery_map_author (cost=0.00..31.19 rows=2559 width=0) (actual time=0.694..0.694 rows=2402 loops=1) Index Cond: ((author)::text ~~ '%研究室%'::text) Planning time: 0.306 ms Execution time: 4.403 ms (7 rows) Time: 5.227 ms

gin_trgm索引的效果好多了

由於pg_trgm的索引是把字符串切成多個3元組，然后使用這些3元組做匹配，所以gin_trgm索引對於少於3個字符（包括漢字）的查詢，只有前綴匹配會走索引

另外，還測試了btree_gin，效果和btree一樣

注意：
gin_trgm要求數據庫必須使用UTF-8編碼

demo_v1 #  \l demo_v1
                               List of databases
  Name   |   Owner   | Encoding |   Collate   |    Ctype    | Access privileges
---------+-----------+----------+-------------+-------------+------------------- demo_v1 | wmpp_user | UTF8 | en_US.UTF-8 | en_US.UTF-8 | 

參考：

http://www.postgres.cn/docs/9.4/pgtrgm.html
http://dba.stackexchange.com/questions/10694/pattern-matching-with-like-similar-to-or-regular-expressions-in-postgresql/10696

 

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原文標題：PostgreSQL LIKE 查詢效率提升實驗

原文地址：http://www.open-open.com/lib/view/open1463100004089.html