Re: [GENERAL] how to import "where exists（subquery）" EXISTS CONDITION performance?

On 2 December 2015 at 20:51, shili <shi_li_1992(at)163(dot)com> 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 sounds like it might be quite out-dated information. Semi joins have
been supported for around 7 years.

See
http://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=e006a24ad152b3faec748afe8c1ff0829699b2e6

However, there are cases where this is still not possible, for example if
the subquery contains a volatile function, or perhaps a LIMIT or OFFSET.

You can view the output from the planner by prefixing your SELECT statement
with EXPLAIN: See
http://www.postgresql.org/docs/current/static/sql-explain.html
This will give you the information you need to see how the query planner
has decided on how your query will be executed.

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David Rowley http://www.2ndQuadrant.com/
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