PostgreSQL LIKE 查询效率提升实验<转>

一、未做索引的查询效率

作为对比，先对未索引的查询做测试

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

很显然都会做全表扫描

二、创建btree索引

PostgreSQL默认索引是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，等于、like的全匹配是用到索引的，like的模糊查询还是全表扫描

三、创建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

由于pg_trgm的索引是把字符串切成多个3元组，然后使用这些3元组做匹配，所以gin_trgm索引对于少于3个字符（包括汉字）的查询，只有前缀匹配会走索引

另外，还测试了btree_gin，效果和btree一样

注意：
gin_trgm要求数据库必须使用UTF-8编码

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 |

参考：

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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原文标题：PostgreSQL LIKE 查询效率提升实验

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