Re: Possible to improve optimisation / index usage based on domain properties of a function

On Thu, Feb 20, 2014 at 3:34 PM, Alban Hertroys <haramrae(at)gmail(dot)com> 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');
>
>
He has already posted that:
*select count(*) from streams where stream_date >= '2013-01-08' and
stream_date < '2013-01-09';*
This would use index

> >
> >> 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.

1. I think Tim is talking about index usage [an index which he has on
stream_date] and not partition

2. Tim, the index usage or non-usage in your two queries would remain same
even if you have a single huge table

3. I believe the partitioning tirgger/rule could re-direct records based on
an expression too [e.g. when date(stream_date)=24-Jan-2014, send it to
Partition_24Jan14]. I am not sure if query planner would go to a particular
partition when we query based on date(stream_date). I need to test this.

Best Regards,

*Sameer Kumar | Database Consultant*

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