Re: Query Optimizer Failure / Possible Bug - Mailing list pgsql-performance

From Hannes Dorbath
Subject Re: Query Optimizer Failure / Possible Bug
Date
Msg-id d293gg$1p0e$1@news.hub.org
Whole thread Raw
In response to Query Optimizer Failure / Possible Bug  (Hannes Dorbath)
Responses Re: Query Optimizer Failure / Possible Bug  (PFC)
List pgsql-performance
hm, a few days and not a single reply :|

any more information needed? test data? simplified test case? anything?


thanks


Hannes Dorbath wrote:
> The query and the corresponding EXPLAIN is at
>
> http://hannes.imos.net/query.txt
>
> I'd like to use the column q.replaced_serials for multiple calculations
> in the SELECT clause, but every time it is referenced there in some way
> the whole query in the FROM clause returning q is executed again.
>
> This doesn't make sense to me at all and eats performance.
>
> If this wasn't clear enough, for every
>
> q.replaced_serials <insert_random_calculation> AS some_column
>
> in the SELECT clause there is new block of
>
> ---------------------------------------------------------------
> ->  Aggregate  (cost=884.23..884.23 rows=1 width=0)
>       ->  Nested Loop  (cost=0.00..884.23 rows=1 width=0)
>             ->  Index Scan using ix_rma_ticket_serials_replace on
>                         rma_ticket_serials rts  (cost=0.00..122.35
>                         rows=190 width=4)
>                   Index Cond: ("replace" = false)
>             ->  Index Scan using pk_serials on serials s
>                           (cost=0.00..3.51 rows=1 width=4)
>                   Index Cond: (s.serial_id = "outer".serial_id)
>                   Filter: ((article_no = $0) AND (delivery_id = $1))
> ---------------------------------------------------------------
>
> in the EXPLAIN result.
>
> For those who wonder why I do this FROM (SELECT...). I was searching for
> a way to use the result of an subselect for multiple calculations in the
> SELECT clause and return that calculation results as individual columns.
>
> I tested a bit further and found out that PG behaves the same in case q
> is a view. This makes me wonder how efficient the optimizer can work
> with views - or even worse - nested views.
>
> Tested and reproduced on PG 7.4.1 linux and 8.0.0 win32.
>
>
> Thanks in advance,
> Hannes Dorbath

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