Re: Very slow select

> Stefano Bargioni writes:
> > SELECT FROM t WHERE a='my string'; works very fast. The same select
> > made on b is very slow (2 or 3 seconds), as if no index exists for
> > column b.
>
> What does EXPLAIN say about the two queries?
>
> regards, tom lane
>

Dear Tom, thanks for your help. Here is the explain results.
As you note, the second select is not index driven. This is why it is slow. Are indexes built correctly / used / when fields contain very large text strings? Can an hash index be useful?
Bye (Merry 2K Xmas!). Stefano

ucc2=> explain select count(*) from document where xtitle='a';
NOTICE: QUERY PLAN:

Aggregate (cost=70.61 rows=759 width=4)
-> Index Scan using xtitle_index on document (cost=70.61 rows=759 width=4)

EXPLAIN
ucc2=> explain select count(*) from document where xothertitles='a';
NOTICE: QUERY PLAN:

Aggregate (cost=26005.44 rows=287195 width=4)
-> Seq Scan on document (cost=26005.44 rows=287195 width=4)

EXPLAIN
ucc2=> \d document
Table = document
+----------------------------------+----------------------------------+-------+
| Field | Type | Length|
+----------------------------------+----------------------------------+-------+
| id | int4 not null default nextval('" | 4 |
| title | text | var |
| xtitle | text | var |
| othertitles | text | var |
| xothertitles | text | var |
| author | text | var |
| xauthor | text | var |
| otherauthors | text | var |
| xotherauthors | text | var |
| subject | text | var |
| xsubject | text | var |
| publisher | text | var |
| xpublisher | text | var |
| collection | text | var |
| xcollection | text | var |
| yearpub | int4 | 4 |
| xyearpub | int4 | 4 |
| locationpub | text | var |
| xlocationpub | text | var |
| languagecode | text | var |
| classification | text | var |
| digest | text | var |
+----------------------------------+----------------------------------+-------+
Indices: digest_document_index
document_id_key
xauthor_index
xcollection_index
xotherauthors_index
xothertitles_index
xpublisher_index
xsubject_index
xtitle_index

indexes where created with
create index xothertitles_index on document (xothertitles);
create index xtitle_index on document (xtitle);
--
Dott. Stefano Bargioni
Biblioteca della Pontificia Universita' della Santa Croce - Roma
<mailto:bargioni(at)usc(dot)urbe(dot)it> <http://www.usc.urbe.it>
Personal web page: <http://www.usc.urbe.it/html/php.script?bargioni>
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