2 Commits (87081b2d35057c4919a8a05a7c52fe87845874d5)

Author	SHA1	Message	Date
Tom Lane	31468d05d8	Dept of better ideas: refrain from creating the planner's placeholder_list ... until vars are distributed to rels during query_planner() startup. We don't really need it before that, and not building it early has some advantages. First, we don't need to put it through the various preprocessing steps, which saves some cycles and eliminates the need for a number of routines to support PlaceHolderInfo nodes at all. Second, this means one less unused plan for any sub-SELECT appearing in a placeholder's expression, since we don't build placeholder_list until after sublink expansion is complete.	17 years ago
Tom Lane	e6ae3b5dbf	Add a concept of "placeholder" variables to the planner. These are variables ... that represent some expression that we desire to compute below the top level of the plan, and then let that value "bubble up" as though it were a plain Var (ie, a column value). The immediate application is to allow sub-selects to be flattened even when they are below an outer join and have non-nullable output expressions. Formerly we couldn't flatten because such an expression wouldn't properly go to NULL when evaluated above the outer join. Now, we wrap it in a PlaceHolderVar and arrange for the actual evaluation to occur below the outer join. When the resulting Var bubbles up through the join, it will be set to NULL if necessary, yielding the correct results. This fixes a planner limitation that's existed since 7.1. In future we might want to use this mechanism to re-introduce some form of Hellerstein's "expensive functions" optimization, ie place the evaluation of an expensive function at the most suitable point in the plan tree.	17 years ago