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Thread: Problems with index-scan on regexp in 8.1

Problems with index-scan on regexp in 8.1

From
Lars Kanis
Date:
We're using Postgres 8.0.2 on SuSE10.0 (64-Bit). Tests on 8.1 beta 4 have 
shown no problems but this one:
 SELECT * FROM mitglieder WHERE lower(vorname::text)='lars'

does a bitmap-index-scan like this:
 Bitmap Heap Scan on mitglieder  (cost=10.68..3770.52 rows=1051 width=226)   Recheck Cond: (lower((vorname)::text) =
'lars'::text)  ->  Bitmap Index Scan on mitgl_lower_namen_idx  (cost=0.00..10.68 
 
rows=1051 width=0)         Index Cond: (lower((vorname)::text) = 'lars'::text)

but a regular expression always results in a seqscan:
 SELECT * FROM mitglieder WHERE lower(vorname::text)~'^lars'
 Seq Scan on mitglieder  (cost=0.00..79703.73 rows=1 width=226)   Filter: (lower((vorname)::text) ~ '^lars'::text)

whereas V8.0.2 does a proper index-scan:
 Index Scan using mitgl_lower_namen_idx on mitglieder  (cost=0.01..18.05 
rows=4 width=225)   Index Cond: ((lower((vorname)::text) >= 'lars'::text) AND 
(lower((vorname)::text) < 'lart'::text))   Filter: (lower((vorname)::text) ~ '^lars'::text)


The use of indexes for regexp is quite important for the search in our 
interactive frontend.


kind regards
Lars Kanis

Re: Problems with index-scan on regexp in 8.1

From
Martijn van Oosterhout
Date:
On Mon, Nov 07, 2005 at 07:50:20AM +0100, Lars Kanis wrote:
> We're using Postgres 8.0.2 on SuSE10.0 (64-Bit). Tests on 8.1 beta 4 have
> shown no problems but this one:
>
>   SELECT * FROM mitglieder WHERE lower(vorname::text)='lars'
>
> does a bitmap-index-scan like this:

Check your locales. For non-ASCII locales the normal shortcuts for
regex optimisation can't apply. Evidently your old installation uses a
different locale from your new one.

You should be able to make this work by declaring your index with
"text_pattern_ops", like so:

CREATE INDEX myindex ON mytable(mycolumn text_pattern_ops);

Hope this helps,
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.

Re: Problems with index-scan on regexp in 8.1

From
Lars Kanis
Date:
Am Montag, 7. November 2005 14:13 schrieb Martijn van Oosterhout:
> On Mon, Nov 07, 2005 at 07:50:20AM +0100, Lars Kanis wrote:
> > We're using Postgres 8.0.2 on SuSE10.0 (64-Bit). Tests on 8.1 beta 4 have
> > shown no problems but this one:
> >
> >   SELECT * FROM mitglieder WHERE lower(vorname::text)='lars'
> >
> > does a bitmap-index-scan like this:
>
> Check your locales. For non-ASCII locales the normal shortcuts for
> regex optimisation can't apply. Evidently your old installation uses a
> different locale from your new one.
>
> You should be able to make this work by declaring your index with
> "text_pattern_ops", like so:
>
> CREATE INDEX myindex ON mytable(mycolumn text_pattern_ops);
>
> Hope this helps,

Thank you much, it helps.
The initdb-locales were different. pattern_ops work quite fine with the
regexps.

So I don't have any complaints to 8.1.

kind regards
Lars Kanis