PostgreSQL LIKE 查询效率提升实验
wuxqing
8年前
<h2>一、未做索引的查询效率</h2> <p>作为对比，先对未索引的查询做测试</p> <pre> <code class="language-sql">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 </code></pre> <pre> <code class="language-sql">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 </code></pre> <pre> <code class="language-sql">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</code></pre> <p>很显然都会做全表扫描</p> <h2>二、创建btree索引</h2> <p>PostgreSQL默认索引是btree</p> <pre> <code class="language-sql">CREATE INDEX ix_gallery_map_author ON gallery_map (author); </code></pre> <pre> <code class="language-sql">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 </code></pre> <pre> <code class="language-sql">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 </code></pre> <pre> <code class="language-sql">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</code></pre> <pre> <code class="language-sql">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 </code></pre> <p>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</p> <h2>三、创建gin索引</h2> <pre> <code class="language-sql">CREATE EXTENSION pg_trgm; CREATE INDEX ix_gallery_map_author ON gallery_map USING gin (author gin_trgm_ops);</code></pre> <pre> <code class="language-sql">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</code></pre> <pre> <code class="language-sql">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</code></pre> <pre> <code class="language-sql">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</code></pre> <p><br> gin_trgm索引的效果好多了</p> <p>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</p> <p>另外，还测试了btree_gin，效果和btree一样</p> <p>注意：<br> gin_trgm要求数据库必须使用UTF-8编码</p> <pre> <code class="language-sql">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 | </code></pre> <p>参考：</p> <p>http://www.postgres.cn/docs/9.4/pgtrgm.html<br> http://dba.stackexchange.com/questions/10694/pattern-matching-with-like-similar-to-or-regular-expressions-in-postgresql/10696</p> <p> </p> <p>本站原创，转载时保留以下信息：</p> <p>本文转自：深度开源（open-open.com）</p> <p>原文标题：<a href="http://www.open-open.com/lib/view/open1463100004089.html">PostgreSQL LIKE 查询效率提升实验</a></p> <p>原文地址：<a href="http://www.open-open.com/lib/view/open1463100004089.html">http://www.open-open.com/lib/view/open1463100004089.html</a></p>