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?
You mean "improve", not "import", right? If you try it out you will find that PostgreSQL often executes an EXISTS clause as a join, so no, SQL statements using EXISTS are not necessarily slow. Still it is often a good idea to use an explicit join instead of EXISTS if possible. Yours, Laurenz Albe -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
