Re: Query Plan far worse in 7.3.2 than 7.2.1 - Mailing list pgsql-performance

"Peter Darley" <pdarley@kinesis-cem.com> writes:
> SELECT COUNT(*) FROM Border_Shop_List  WHERE NOT EXISTS (SELECT Foreign_Key=
>  FROM Sample WHERE Foreign_Key=3D'Quantum_' || Border_Shop_List.Assignment_=
> ID || '_' || Assignment_Year || '_' || Evaluation_ID)

What's the datatype of Foreign_Key?

I'm betting that it's varchar(n) or char(n).  The result of the ||
expression is text, and so the comparison can't use a varchar index
unless you explicitly cast it to varchar:
    WHERE Foreign_Key = ('Quantum_' || ... || Evaluation_ID)::varchar

I think 7.2 had some kluge in it that would allow a varchar index to be
used anyway, but we took out the kluge because it was semantically wrong
(it would also allow use of a char(n) index in place of a text
comparison, which alters the semantics...)

            regards, tom lane