postgresql 检查列中是否存在值
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Check value if exists in column
提问by Perlos
I'd like to know how to maximize speed when querying for the presence of a varcharvalue in a column in a specific table. I don't need to know where it is, or how many occurrences there are, I just want a true/false. Of course, the column has an index.
我想知道在查询varchar特定表的列中是否存在值时如何最大限度地提高速度。我不需要知道它在哪里，或者出现了多少次，我只想要一个true/ false。当然，该列有一个索引。
Now, I have this:
现在，我有这个：
SELECT exists (SELECT 1 FROM table WHERE column = <value> LIMIT 1);
回答by Mike Sherrill 'Cat Recall'
EXISTS should normally return as soon as the subquery finds one row that satisfies its WHERE clause. So I think your query is as fast as you can make it.
EXISTS 通常应该在子查询找到满足其 WHERE 子句的行后立即返回。所以我认为您的查询尽可能快。
I was a little surprised that LIMIT 1seems to always speed up the query very slightly. I didn't expect that. You can see the effect with EXPLAIN ANALYZE.
我有点惊讶，这LIMIT 1似乎总是略微加快查询速度。我没想到。你可以看到效果EXPLAIN ANALYZE。
EXPLAIN ANALYZE
SELECT exists (SELECT 1 FROM table WHERE column = <value> LIMIT 1);
回答by blindguy
I have had recent success with the following in some cases:
在某些情况下，我最近在以下方面取得了成功：
SELECT count(1) > 0
WHERE column = <value>
