Re: Does index based on text with 'en_US.utf8' collation support like query?

Hello,

On Dec 25, 2012, at 1:37 PM, Haifeng Liu  wrote:

> 
> Hi,
> 
> I found that queries like "select * from tbl where col1 like 'abc%'" will use index only when the col1(and the index) use 'C' collation. If I use 'default' which is 'en_US.utf8' in my database, the equation condition will use the index but like query will not.

This is a documented behavior, LIKE with a default operator class only works with C locale.

> 
> I also tried to specify the collation explicitly: like ('abc%' collate 'en_US.utf8'), not work too.
> 
> Is that only 'C' collation support 'like' query or I missed something in my sql? Or do I need to install something additional to support index on other collation?

You need to add a new index with a special operator class called 'text_pattern_ops' (or 'varchar_pattern_ops), i.e:
CREATE INDEX col1_pattern_idx ON tbl(col1 text_pattern_ops);
 
This index will be used in LIKE queries under non-C locale. You also need to preserve your existing default (aka text_ops) indexes if you want comparison or inequality operations on the col1 column to be indexed. 

Refer to http://www.postgresql.org/docs/current/static/indexes-opclass.html for more details.

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