On Fri, Jun 8, 2018 at 8:52 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: > David Rowley <david.row...@2ndquadrant.com> writes: >> On 8 June 2018 at 03:43, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> Maybe there's something I'm missing here, but I sort of hoped that this >>> patch would nuke all the special-case code for Params in this area. >>> Why is there any need to distinguish them from other stable expressions? > >> We need to know which Params exist in the Expr as if there are no >> Params, or only external Params, then we can run-time prune during >> startup of the executor. > > This does not refute my question. Why doesn't the same logic apply > to any stable expression? That is, ISTM a Param is a special case > of that. >
+1. I don't think we need to perform pruning at the start of execution, but we could fold all the stable expressions to constants at that time. The PARAM_EXECs can not be folded into constant at execution start since those not assigned any values yet. AFAIU expressions, within a given node, with those parameters can be folded into constants (if possible) during ExecRewind() on that node. We have to perform pruning just before the (Merge)Append node scan starts. -- Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company
