a5db8e12b3
eval_const_expressions() can replace CaseTestExprs with constants when the surrounding CASE's test expression is a constant. This confuses ruleutils.c's heuristic for deparsing simple-form CASEs, leading to Assert failures or "unexpected CASE WHEN clause" errors. I had put in a hack solution for that years ago (see commit 514ce7a331c5bea8e55b106d624e55732a002295 of 2006-10-01), but bug #5794 from Peter Speck shows that that solution failed to cover all cases. Fortunately, there's a much better way, which came to me upon reflecting that Peter's "CASE TRUE WHEN" seemed pretty redundant: we can "simplify" the simple-form CASE to the general form of CASE, by simply omitting the constant test expression from the rebuilt CASE construct. This is intuitively valid because there is no need for the executor to evaluate the test expression at runtime; it will never be referenced, because any CaseTestExprs that would have referenced it are now replaced by constants. This won't save a whole lot of cycles, since evaluating a Const is pretty cheap, but a cycle saved is a cycle earned. In any case it beats kluging ruleutils.c still further. So this patch improves const-simplification and reverts the previous change in ruleutils.c. Back-patch to all supported branches. The bug exists in 8.1 too, but it's out of warranty.
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