| From: | Ron <ronljohnsonjr(at)gmail(dot)com> |
|---|---|
| To: | pgsql-general(at)lists(dot)postgresql(dot)org |
| Subject: | Re: bug or lacking doc hint |
| Date: | 2023-06-25 16:24:49 |
| Message-ID: | 20ce23c6-3c35-e5ce-b5bd-edb3371786a2@gmail.com |
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| Lists: | pgsql-general |
On 6/25/23 10:01, Marc Millas wrote:
> Hi,
>
> I have had a perf (++) pb with a join plan cf the pb with join plan thread.
> I did simplify the thing up to when its a simple join between a 15M lines
> table and a 30k lines table.
> if I put in the on part something like table1.a=table2.b, Postgres does
> the job in around 1 seconde.
>
> if in the on part of the join I write table1.a=table2.b OR
> substr(table1.c,x,x) =table2.d
That left side SUBSTR() can't be good for the query optimizer. (Or I'm old
and still using habits picked up 25 years ago on a different RDBMS.)
Maybe a function index on table1 would help.
--
Born in Arizona, moved to Babylonia.
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