Re: Evaluation of PG performance vs MSDE/MSSQL 2000 (not 2005)

Robert Bernabe wrote:
> In a nutshell it seems that MS SQL allows bad T-SQL code by optimizing and ignoring redundant/useless from and where clauses in an update statement whereas plpgsql will execute exactly what the code is asking it to do...
>
> We had several update instances in the T-SQL code that looked like this :
>
> update "_tbl_tmp2"
> set "LongBackPeriod" = (select count ("EPeriod") from "_tbl_tmp1" where "_tbl_tmp1"."Row" = "_tbl_tmp2"."Row");
> --------------------------------------------------
> from "_tbl_tmp2" tmp2, "_tbl_tmp1" tmp1
> where tmp2."Row" = tmp1."Row";
> ---------------------------------------------------

Those lines are not totally useless from DB point of view. If there is
no rows that match the join, the WHERE clause will be false, and no rows
will be updated. So I'm sure MS SQL doesn't ignore those lines, but does
use a more clever plan. Perhaps it stops processing the join as soon as
is finds a match, while we perform the whole join, for example.

> In T-SQL, the performance is the same whether the last two lines are there or not...
>
> In plpgsql, this is not the case the from and where clauses are not necessary and probably creates an internal (rather useless and time consuming) inner join in plpgsql which accounts for the original performance issue.

You can check the access plan with EXPLAIN.

> I'm happy (actually ecstatic) to report that Win2kPro + PG performance is slightly faster than Win2kPro + MSSQL/MSDE.
>
> Linux(FC7) + PG 8.x performance seems to be 3x faster than Win2KPro + MSSQL/MSDE for our stored functions.
>
> Thanks for all the help! Am a believer now. :)

Nice to hear :).

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com