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Re: Query performance issue with 8.0.0beta1

From: Russell Smith <mr-russ(at)pws(dot)com(dot)au>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Query performance issue with 8.0.0beta1
Date: 2004-08-27 08:48:08
Message-ID: 200408271848.08696.mr-russ@pws.com.au (view raw or flat)
Thread:
Lists: pgsql-performance
7.4.2
>  Aggregate  (cost=46817.89..46817.89 rows=1 width=0) (actual time=401.216..401.217 rows=1 loops=1)
>    ->  Index Scan using snsdata_codpar on "SNS_DATA"  (cost=0.00..46817.22 rows=268 width=0) (actual time=165.948..400.258 rows=744 loops=1)
>          Index Cond: (("Cod_Par")::text = '17476'::text)
>          Filter: (("Data_Arrivo_Campione" >= '2004-01-01 00:00:00'::timestamp without time zone) AND ("Data_Arrivo_Campione" <= '2004-01-31 23:59:59'::timestamp without time zone))
>  Total runtime: 401.302 ms
> 
Row counts are out by a factor of 3, on the low side. so the planner will guess index is better, which it is.

> ***while on 8.0.0***
>  Aggregate  (cost=93932.91..93932.91 rows=1 width=0) (actual time=14916.371..14916.371 rows=1 loops=1)
>    ->  Seq Scan on "SNS_DATA"  (cost=0.00..93930.14 rows=1108 width=0) (actual time=6297.152..14915.330 rows=744 loops=1)
>          Filter: (("Data_Arrivo_Campione" >= '2004-01-01 00:00:00'::timestamp without time zone) AND ("Data_Arrivo_Campione" <= '2004-01-31 23:59:59'::timestamp without time zone) AND (("Cod_Par")::text = '17476'::text))
>  Total runtime: 14916.935 ms
Planner guesses that 1108 row should be returned, which is out by less, but on the high side.
Big question is given there are 2M rows, why does returning 1108 rows, less than 1% result in a sequence scan.
Usually the selectivity on the index is bad, try increasing the stats target on the column.

I know 8.0 has new stats anaylsis code, which could be effecting how it choses the plan. But it would still
require a good amount of stats to get it to guess correctly.

Increase stats and see if the times improve.

> 
> And I if disable the seqscan
> SET enable_seqscan = false;
> 
> I get the following:
> 
> Aggregate  (cost=158603.19..158603.19 rows=1 width=0) (actual time=4605.862..4605.863 rows=1 loops=1)
>    ->  Index Scan using snsdata_codpar on "SNS_DATA"  (cost=0.00..158600.41 rows=1108 width=0) (actual time=2534.422..4604.865 rows=744 loops=1)
>          Index Cond: (("Cod_Par")::text = '17476'::text)
>          Filter: (("Data_Arrivo_Campione" >= '2004-01-01 00:00:00'::timestamp without time zone) AND ("Data_Arrivo_Campione" <= '2004-01-31 23:59:59'::timestamp without time zone))
>  Total runtime: 4605.965 ms
> 
> The total runtime is bigger (x10 !!) than the old one.
Did you run this multiple times, or is this the first time.  If it had to get the data off disk it will be slower.
Are you sure that it's coming from disk in this and the 7.4 case? or both from memory.
If 7.4 is from buffer_cache, or kernel_cache, and 8.0 is from disk you are likely to get A LOT slower.

> 
> The memory runtime parameters are 
> shared_buffer = 2048
> work_mem = sort_mem = 2048
> 
[ snip ]

> The table has 2M of records
> Can it be a datatype conversion issue?
That should not be an issue in 8.0, at least for the simple type conversions.  like int8 to int4.
I'm not 100% sure which ones were added, and which were not, but the query appears to cast everything correctly anyway.

> Can it be depend on the the type of restore (with COPY commands)?
Shouldn't and VACUUM FULL ANALYZE will make the table as small as possible.  The row order may be different
on disk, but the planner won't know that, and it's a bad plan causing the problem.

> I have no idea.
> 
> Thanks in advance!
> Reds
> 
Regards

Russell Smith.

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Subject: Re: Equivalent praxis to CLUSTERED INDEX?
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