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The previous coding saved pointers into the partitioned table's relcache entry, but then closed the relcache entry, causing those pointers to nominally become dangling. Actual trouble would be seen in the field only if a relcache flush occurred mid-query, but that's hardly out of the question. While we could fix this by copying all the data in question at query start, it seems better to just hold the relcache entry open for the whole query. While at it, improve the handling of support-function lookups: do that once per query not once per pruning test. There's still something to be desired here, in that we fail to exploit the possibility of caching data across queries in the fn_extra fields of the relcache's FmgrInfo structs, which could happen if we just used those structs in-place rather than copying them. However, combining that with the possibility of per-query lookups of cross-type comparison functions seems to require changes in the APIs of a lot of the pruning support functions, so it's too invasive to consider as part of this patch. A win would ensue only for complex partition key data types (e.g. arrays), so it may not be worth the trouble. David Rowley and Tom Lane Discussion: https://postgr.es/m/17850.1528755844@sss.pgh.pa.us
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