On 10/19/06, Jens Schipkowski <jens.schipkowski@apus.co.at> wrote:
> // select finds out which one has not an twin
> // a twin is defined as record with the same attr* values
> // decreases speed over time until timeout by postgresql
> SELECT *
> FROM tbl_reg reg
> WHERE register <> loc1 AND
> idreg NOT IN
> (
> SELECT reg.idreg
> FROM tbl_reg reg, tbl_reg regtwin
> WHERE regtwin.register = 1 AND
> regtwin.type <> 20 AND
> reg.attr1 = regtwin.attr1 AND
> reg.attr2 = regtwin.attr2 AND
> reg.attr3 = regtwin.attr3 AND
> reg.attr4 = regtwin.attr4 AND
> reg.attr5 = regtwin.attr5 AND
> reg.attr6 = regtwin.attr6 AND
> reg.idreg <> regtwin.idreg AND
> reg.register = 2
> );
[...]
> We have the problem, that we cannot see any potential to improve SQL
> statements. Indexing the attr* columns seems not to be an solution,
> because the data mustn't be unique (twins) and changes really often so
> reindexing will took too long.
1. your database design is the real culprit here. If you want things
to run really quickly, solve the problem there by normalizing your
schema. denomalization is the root cause of many, many, problems
posted here on this list.
2. barring that, the above query will run fastest by creating
multi-column indexes on regtwin (attr*) fields. and reg(attr*). the
real solution to problems like this is often proper idnexing,
especially multi column. saying indexes take to long to build is like
saying: 'i have a problem, so i am going to replace it with a much
worse problem'.
3. try where exists/not exists instead of where in/not in
merlin