On 20 Feb 2014, at 7:21, Sameer Kumar <sameer.kumar@xxxxxxxxxx> wrote: > If however, I was to provide the below query, it uses a sequential scan based plan. The planner is unable to utilise any indexes because it can’t know what the function is going to return – thus unable to constrain the range at the time of planning the execution. > > select count(*) from streams where date(stream_date) = ‘2013-01-08’; The inverse of that expression, if it’s possible to formulate one, would most likely use the index though: select count(*) from streams where stream_date = inv_date(‘2013-01-08’); > >> I’m wondering if we could build into postgres some level of metadata regarding the properties of a function, such that the optimiser could filter against the range of values that the function is expected to return. >> >> In this case, it could deduce that the date function will only ever return a value for stream_date to within a certain maximum and minimum range. >> Thus the planner could scan the index for all values of stream_date falling within +/- 24 hours of the right operand, and then check/re-check the results. >> > If you can't go for the smarter query, go for more optimum index by "expression based index" > > http://www.postgresql.org/docs/9.1/static/indexes-expressional.html AFAIK, you can’t use expression based indexes to partition a table, so that won’t help the OP much. Alban Hertroys -- If you can't see the forest for the trees, cut the trees and you'll find there is no forest. -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
