[HACKERS] what exactly is a PlaceHolderVar?
I can't find any good documentation of this in the source tree anywhere. placeholder.c just says: *PlaceHolderVar and PlaceHolderInfo manipulation routines and placeholder.h says: *prototypes for optimizer/util/placeholder.c. ...which is less than informative. The commit message that introduced them has a few details: 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). ...but I'm still having a hard time wrapping my head around it. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise Postgres Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] what exactly is a PlaceHolderVar?
Robert Haas robertmh...@gmail.com writes: ...but I'm still having a hard time wrapping my head around it. The fundamental point is to be able to force a value to go to NULL when outer-join logic says it ought to. Consider CREATE VIEW foo AS SELECT x,y,'zed' FROM bar; SELECT * FROM baz LEFT JOIN foo ON (baz.a = foo.x); If you try to flatten the view then you end up with a constant 'zed' that needs to be replaced by NULL whenever baz.a hasn't got a match in bar.x. There's no way to make a constant go to NULL though: it's a constant, n'est-ce pas? Instead, we have the idea of an expression PlaceHolderVar(foo, 'zed'). This will go to null if variables from foo ought to go to null. Otherwise it produces 'zed'. Sort of an anti-COALESCE. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
