Are indexes used with LIKE?

On Mon, Jan 23, 2006 at 08:00:01PM +0100, Kovcs Pter wrote:

Are indexes on VARCHAR columns used with the LIKE operator, and if so,
how efficiently are they used?

I can imagine that using indexes can be easy with the starting literal
characters up to the first percent sign such as in:

LIKE 'ZOE%QQWE%'

But, after the first % sign, things can get more difficult.

The planner can use an index on the starting literal characters;
how "difficult" the query becomes after that depends on how
discriminating those initial characters are. If values matching
the initial characters comprise a small fraction of the table then
the query will probably use an index and be fast, but if they
comprise a large fraction of the table, or if the search string
starts with a wildcard, then you'll get a sequential scan, which
might be slow.

The reason I am asking is that we are thinking about discriminating
between rows of a table based on a VARCHAR column containing various
one-character "flags". We could then use the LIKE operator for
formulating filter conditions.

Have you considered putting each flag in a separate column and
indexing those columns? If you're using 8.1 the planner would
probably use bitmap index scans and come up with a fast plan
regardless of which columns you restrict on. And performance issues
aside, some people would consider that a better design. However,
a disadvantage might be that your queries would be more complex.

--
Michael Fuhr

Thank you for the answer!

Sure, the possiblity of having a separate column for each flag was
considered, but a common columnn is preferred -- I do not remember
exactly why. (I do not directly make that decision.) I guess the main
reason is that adding new columns to the table complicates the upgrade
procedure with our existing customer base.

Thank you again!

Peter

Michael Fuhr wrote: