Ignore PlaceHolderVars when looking up statistics When looking up statistical data about an expression, we failed to look through PlaceHolderVar nodes, treating them as opaque. This could prevent us from matching an expression to base columns, index expressions, or extended statistics, as examine_variable() relies on strict structural matching.
As a result, queries involving PlaceHolderVar nodes often fell back to default selectivity estimates, potentially leading to poor plan choices. This patch updates examine_variable() to strip PlaceHolderVars before analysis. This is safe during estimation because PlaceHolderVars are transparent for the purpose of statistics lookup: they do not alter the value distribution of the underlying expression. To minimize performance overhead on this hot path, a lightweight walker first checks for the presence of PlaceHolderVars. The more expensive mutator is invoked only when necessary. There is one ensuing plan change in the regression tests, which is expected and demonstrates the fix: the rowcount estimate becomes much more accurate with this patch. Back-patch to v18. Although this issue exists before that, changes in this version made it common enough to notice. Given the lack of field reports for older versions, I am not back-patching further. Reported-by: Haowu Ge <[email protected]> Author: Richard Guo <[email protected]> Discussion: https://postgr.es/m/[email protected] Backpatch-through: 18 Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/559f9e90dbbd5d72b1da802703317913280c5080 Modified Files -------------- src/backend/utils/adt/selfuncs.c | 93 +++++++++++++++++++++++++++++++++----- src/test/regress/expected/join.out | 27 ++++++++++- src/test/regress/sql/join.sql | 10 ++++ 3 files changed, 116 insertions(+), 14 deletions(-)
