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If an index has both returnable and non-returnable columns, and one of the non-returnable columns is an expression using a Var that is in a returnable column, then a query returning that expression could result in an index-only scan plan that attempts to read the non-returnable column, instead of recomputing the expression from the returnable column as intended. To fix, redefine the "indextlist" list of an IndexOnlyScan plan node as containing null Consts in place of any non-returnable columns. This solves the problem by preventing setrefs.c from falsely matching to such entries. The executor is happy since it only cares about the exposed types of the entries, and ruleutils.c doesn't care because a correct plan won't reference those entries. I considered some other ways to prevent setrefs.c from doing the wrong thing, but this way seems good since (a) it allows a very localized fix, (b) it makes the indextlist structure more compact in many cases, and (c) the indextlist is now a more faithful representation of what the index AM will actually produce, viz. nulls for any non-returnable columns. This is easier to hit since we introduced included columns, but it's possible to construct failing examples without that, as per the added regression test. Hence, back-patch to all supported branches. Per bug #17350 from Louis Jachiet. Discussion: https://postgr.es/m/17350-b5bdcf476e5badbb@postgresql.org |
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Documentation concerning how to run these regression tests and interpret the results can be found in the PostgreSQL manual, in the chapter "Regression Tests".