Re: Super PathKeys (Allowing sort order through precision loss functions)

On 1 November 2018 at 12:24, Andres Freund <andres@anarazel.de> wrote:
> FWIW, I kind of wonder if we built proper infrastructure to allow to
> make such inferrences from function calls, whether it could also be made
> to support the transformation of LIKEs into indexable <= >= clauses.

Perhaps, but I doubt it would be the same function to do both.  Surely
I need something that accepts details about the function call as
arguments and returns an Expr * of the argument that we can derive the
order of the return value from, or NULL.  I think the transformation
you need might be more along the lines of returning a List * of quals
that can substitute an OpExpr containing a function call. I'm not that
clear on how we'd know the new quals were better than the existing
ones.  For example extract('year', dt)  = 2018 could be transformed to
dt >= '2018-01-01' AND dt < '2019-01-01', but how would we know that
was better. There might be an index on extract('year', dt).

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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