Re: need to repeat the same condition on joined tables in order to choose the proper plan

Svetlin Manavski <svetlin(dot)manavski(at)gmail(dot)com> writes:
> I am really surprised to see that the planner needs me to explicitly specify
> the same condition twice like this:

> SD.detectorid = SS.detectorid
> and SD.sessionid = SS.id
> and SD.detectorid = 1
> and SD.sessionid >= 1001000000000::INT8 and SD.sessionid <=
> 2001000000000::INT8
> and SS.detectorid = 1
> and SS.id >= 1001000000000::INT8 and SS.id <= 2001000000000::INT8

The planner does infer implied equalities, eg, given A = B and B = C
it will figure out that A = C. What you are asking is for it to derive
inequalities, eg infer A < C from A = B and B < C. That would be
considerably more work for considerably less reward, since the sort of
situation where this is helpful doesn't come up very often. On balance
I don't believe it's a good thing for us to do: I think it would make
PG slower on average because on most queries it would just waste time
looking for this sort of situation.

(In this example, the SS.detectorid = 1 clause is in fact unnecessary,
since the planner will infer it from SD.detectorid = SS.detectorid and
SD.detectorid = 1. But it won't infer the range conditions on SS.id
from the range conditions on SD.sessionid or vice versa.)

regards, tom lane