On 12/29/23 17:27, Corey Huinker wrote: > But maybe it's enough to just do what you did - if we get an MCELEM > slot, can it ever contain anything else than array of elements of the > attribute array type? I'd bet that'd cause all sorts of issues, no? > > > Thanks for the explanation of why it wasn't working for me. Knowing that > the case of MCELEMÂ + is-array-type is the only case where we'd need to > do that puts me at ease. >
But I didn't claim MCELEM is the only slot where this might be an issue. I merely asked if a MCELEM slot can ever contain an array with element type different from the "original" attribute. After thinking about this a bit more, and doing a couple experiments with a trivial custom data type, I think this is true: 1) MCELEM slots for "real" array types are OK I don't think we allow "real" arrays created by users directly, all arrays are created implicitly by the system. Those types always have array_typanalyze, which guarantees MCELEM has the correct element type. I haven't found a way to either inject my custom array type or alter the typanalyze to some custom function. So I think this is OK. 2) I'm not sure we can extend this regular data types / other slots For example, I think I can implement a data type with custom typanalyze function (and custom compute_stats function) that fills slots with some other / strange stuff. For example I might build MCV with hashes of the original data, a CountMin sketch, or something like that. Yes, I don't think people do that often, but as long as the type also implements custom selectivity functions for the operators, I think this would work. regards -- Tomas Vondra EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
