Tom Lane f8abb0f5e1 postgres_fdw: suppress casts on constants in limited cases. ... When deparsing an expression of the form "remote_var OP constant", we'd normally apply a cast to the constant to make sure that the remote parser thinks it's of the same type we do. However, doing so is often not necessary, and it causes problems if the user has intentionally declared the local column as being of a different type than the remote column. A plausible use-case for that is using text to represent a type that's an enum on the remote side. A comparison on such a column will get shipped as "var = 'foo'::text", which blows up on the remote side because there's no enum = text operator. But if we simply leave off the explicit cast, the comparison will do exactly what the user wants. It's possible to do this without major risk of semantic problems, by relying on the longstanding parser heuristic that "if one operand of an operator is of type unknown, while the other one has a known type, assume that the unknown operand is also of that type". Hence, this patch leaves off the cast only if (a) the operator inputs have the same type locally; (b) the constant will print as a string literal or NULL, both of which are initially taken as type unknown; and (c) the non-Const input is a plain foreign Var. Rule (c) guarantees that the remote parser will know the type of the non-Const input; moreover, it means that if this cast-omission does cause any semantic surprises, that can only happen in cases where the local column has a different type than the remote column. That wasn't guaranteed to work anyway, and this patch should represent a net usability gain for such cases. One point that I (tgl) remain slightly uncomfortable with is that we will ignore an implicit RelabelType when deciding if the non-Const input is a plain Var. That makes it a little squishy to argue that the remote should resolve the Const as being of the same type as its Var, because then our Const is not the same type as our Var. However, if we don't do that, then this hack won't work as desired if the user chooses to use varchar rather than text to represent some remote column. That seems useful, so do it like this for now. We might have to give up the RelabelType-ignoring bit if any problems surface. Dian Fay, with review and kibitzing by me Discussion: https://postgr.es/m/C9LU294V7K4F.34LRRDU449O45@lamia		2021-11-12 11:50:47 -05:00
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postgres_fdw.out	postgres_fdw: suppress casts on constants in limited cases.	2021-11-12 11:50:47 -05:00