Re: Performance issues with v18 SQL-language-function changes

On Sun, Apr 13, 2025 at 3:23 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> create function fx(p_summa bigint) returns text immutable strict
> return ltrim(to_char(p_summa, '999 999 999 999 999 999 999 999'));
>
> explain analyze select fx(i) from generate_series(1,1000000) as i(i);
>
> you arrive at the rude discovery that 0dca5d68d is about 50% slower
> than 0dca5d68d^, because the old implementation builds a plan for fx()
> only once and then re-uses it throughout the query.

I agree that we should do something about this. I haven't reviewed
your patches but the approach sounds broadly reasonable.

--
Robert Haas
EDB: http://www.enterprisedb.com