Re: [HACKERS] Subselects are in CVS...

Added to backend/optimizer/plan/README.

> 
> This is some implementation notes and opened issues...
> 
> First, implementation uses new type of parameters - PARAM_EXEC - to deal
> with correlation Vars. When query_planner() is called, it first tries to
> replace all upper queries Var referenced in current query with Param of 
> this type. Some global variables are used to keep mapping of Vars to 
> Params and Params to Vars. 
> 
> After this, all current query' SubLinks are processed: for each SubLink 
> found in query' qual union_planner() (old planner() function) will be 
> called to plan corresponding subselect (union_planner() calls 
> query_planner() for "simple" query and supports UNIONs). After subselect 
> are planned, optimizer knows about is this correlated, un-correlated or 
> _undirect_ correlated (references some grand-parent Vars but no parent 
> ones: uncorrelated from the parent' point of view) query. 
> 
> For uncorrelated and undirect correlated subqueries of EXPRession or 


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