Re: Very slow select
От | Stefano Bargioni |
---|---|
Тема | Re: Very slow select |
Дата | |
Msg-id | 3A4327EB.701500F@usc.urbe.it обсуждение исходный текст |
Ответ на | Very slow select (Stefano Bargioni <bargioni@usc.urbe.it>) |
Ответы |
Re: Very slow select
(Tom Lane <tgl@sss.pgh.pa.us>)
|
Список | pgsql-general |
> 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 fieldscontain 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@usc.urbe.it> <http://www.usc.urbe.it> Personal web page: <http://www.usc.urbe.it/html/php.script?bargioni> --- "Si apud bibliothecam hortulum habes, nihil deerit" (Cicerone) ---
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