Обсуждение: Query is slower
Hi I have a query using "like" operator (select * from name like 'JOHN%'), but the table has about 500 hundred records. The has a index (create index ixcontract_name on contract (name)) , but it is very slow because it is not using index. How do I do the query to use index? Sandro -- *Sandro Joel Eller* Analista/Programador Senior www.tecsoft.com.br sandro@tecsoft.com.br
On Wed, 1 Dec 2004, Sandro Joel Eller wrote: > I have a query using "like" operator (select * from name like 'JOHN%'), > but the table has about 500 hundred records. The has a index (create > index ixcontract_name on contract (name)) , but it is very slow because > it is not using index. How do I do the query to use index? Well, there are two possibilities. The more likely one is that the database is not in "C" locale so it won't use the index described (it would need an index with a <type>_pattern_ops operator class). If this is the case, the two options are making the pattern_ops index or re-initdb with "C" locale. The other, less likely, option is that it's expecting a large percentage of rows to match which you can check with EXPLAIN.
On Wed, Dec 01, 2004 at 05:26:59PM -0200, Sandro Joel Eller wrote: > I have a query using "like" operator (select * from name like 'JOHN%'), > but the table has about 500 hundred records. The has a index (create > index ixcontract_name on contract (name)) , but it is very slow because > it is not using index. How do I do the query to use index? How do you know the query doesn't use an index and that using an index would be faster? Did you run EXPLAIN ANALYZE? If so, then please post the output. It might also be useful to see the difference between a sequential scan and an index scan. Try this: EXPLAIN ANALYZE SELECT * FROM contract WHERE name LIKE 'JOHN%'; SET enable_seqscan TO off; EXPLAIN ANALYZE SELECT * FROM contract WHERE name LIKE 'JOHN%'; If the second query still does a sequential scan then you might have a data type mismatch. Or, if the search string begins with % or _, then the query won't be able to use an index. -- Michael Fuhr http://www.fuhr.org/~mfuhr/