Compare collations before merging UNION operations. In the dim past we figured it was okay to ignore collations when combining UNION set-operation nodes into a single N-way UNION operation. I believe that was fine at the time, but it stopped being fine when we added nondeterministic collations: the semantics of distinct-ness are affected by those. v17 made it even less fine by allowing per-child sorting operations to be merged via MergeAppend, although I think we accidentally avoided any live bug from that.
Add a check that collations match before deciding that two UNION nodes are equivalent. I also failed to resist the temptation to comment plan_union_children() a little better. Back-patch to all supported branches (v13 now), since they all have nondeterministic collations. Discussion: https://postgr.es/m/3605568.1731970...@sss.pgh.pa.us Branch ------ REL_14_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/44a4a521d03a0033fca696b5fa4725a436018ae6 Modified Files -------------- src/backend/optimizer/prep/prepunion.c | 23 +++++++++++------------ 1 file changed, 11 insertions(+), 12 deletions(-)
