Re: A very long running query....

[Ioannis Anagnostopoulos <ioannis@xxxxxxxxxx>]

On 21/07/2012 17:58, Tom Lane wrote:

[ Please try to trim quotes when replying.  People don't want to re-read
  the entire thread in every message. ]

Ioannis Anagnostopoulos <ioannis@xxxxxxxxxx> writes:

On 21/07/2012 10:16, Marc Mamin wrote:

isn't the first test superfluous here ?

where extract('day' from message_copies.msg_date_rec) = 17
and date_trunc('day', message_copies.msg_date_rec) = '2012-07-17'

No because it is used to select a partition. Otherwise it will go 
through the whole hierarchy...

You're using extract(day...) to define partitions?  You might want to
rethink that.  The planner has got absolutely no intelligence about
the behavior of extract, and in particular doesn't realize that the
date_trunc condition implies the extract condition; so that's another
part of the cause of the estimation error here.

What's usually recommended for partitioning is simple equality or
range constraints, such as "msg_date_rec >= 'date1' AND
msg_date_rec < 'date2'", which the planner does have a fair amount
of intelligence about.

Now, you can generalize that to equality or range constraints using
an _expression_; for instance there'd be no problem to partition on
date_trunc('day', msg_date_rec) rather than msg_date_rec directly,
so long as your queries always use that same _expression_.  But you
should not expect that the planner can deduce very much about the
correlations between results of different functions.

			regards, tom lane

I think you got this wrong here. If you see the query again you will see that I do use equality. The problem is that my "equality" occurs
by extracting the date from the msg_date_rec column. To put it in other words, for not using the "extract" I should have an additional
column only with the "date" number to perform the equality. Don't you feel that this is not right since I have the actual date? The constrain
within the table  that defines the partition is as follows:

CONSTRAINT message_copies_wk0_date CHECK (date_part('day'::text, msg_date_rec) >= 1::double precision AND date_part('day'::text, msg_date_rec) <= 7::double precision)

I see not problem at this. The planner gets it right and "hits" the correct table every time. So unless if there is a technique here that I completely miss,
where is the problem?


Regards
Yiannis