On 12/1/2015 11:51 PM, shili wrote:
I had saw this sentence: SQL statements that use the EXISTS condition in PostgreSQL are very inefficient since the sub-query is RE-RUN for EVERY row in the outer query's table. There are more efficient ways to write most queries, that do not use the EXISTS condition. So,I want to know how PostgreSQL to implement the EXISTS condition? Is that sentence true? and,if that is true,are there any methods to import the performance of the EXISTS condition?
that would depend on the specific query with which you're having performance problems.
start with EXPLAIN ANALYZE SELECT ....restofyourquery....; to see whats going on
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