Re: [PERFORM] performance drop when function argument is evaluated in WHERE clause

Gerardo Herzig <gherzig(at)fmed(dot)uba(dot)ar> writes:
> Hi all. I have a function that uses a "simple" select between 3 tables. There is a function argument to help choose how a WHERE clause applies. This is the code section:
> select * from....
> [...]
> where case $3
> when 'I' then [filter 1]
> when 'E' then [filter 2]
> when 'P' then [filter 3]
> else true end

> When the function is called with, say, parameter $3 = 'I', the funcion run in 250ms,
> but when there is no case involved, and i call directly "with [filter 1]" the function runs in 70ms.

> Looks like the CASE is doing something nasty.
> Any hints about this?

Don't do it like that. You're preventing the optimizer from understanding
which filter applies. Better to write three separate SQL commands
surrounded by an if/then/else construct.

(BTW, what PG version is that? I would think recent versions would
realize that dynamically generating a plan each time would work around
this. Of course, that approach isn't all that cheap either. You'd
probably still be better off splitting it up manually.)

regards, tom lane