PostgreSQL LIKE 查詢效率提升實驗
阿新 • • 發佈：2019-01-30
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
----------------------------------------------------------------------------------------------------------------- EXPLAIN ANALYZE select * from gallery_map where 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、建立btree索引
PostgreSQL預設索引是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 msEXPLAIN 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、建立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 msEXPLAIN 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 msEXPLAIN 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的索引是把字串切成多個3元組，然後使用這些3元組做匹配，所以gin_trgm索引對於少於3個字元（包括漢字）的查詢，只有字首匹配會走索引
另外，還測試了btree_gin，效果和btree一樣
注意：
gin_trgm要求資料庫必須使用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
本站原創，轉載時保留以下資訊：
本文轉自：深度開源（open-open.com）