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Revalidation of a plancache entry (after a cache invalidation event) requires acquiring a snapshot. Normally that is harmless, but not if the cached statement is one that needs to run without acquiring a snapshot. We were already aware of that for TransactionStmts, but for some reason hadn't extrapolated to the other statements that PlannedStmtRequiresSnapshot() knows mustn't set a snapshot. This can lead to unexpected failures of commands such as SET TRANSACTION ISOLATION LEVEL. We can fix it in the same way, by excluding those command types from revalidation. However, we can do even better than that: there is no need to revalidate for any statement type for which parse analysis, rewrite, and plan steps do nothing interesting, which is nearly all utility commands. To mechanize this, invent a parser function stmt_requires_parse_analysis() that tells whether parse analysis does anything beyond wrapping a CMD_UTILITY Query around the raw parse tree. If that's what it does, then rewrite and plan will just skip the Query, so that it is not possible for the same raw parse tree to produce a different plan tree after cache invalidation. stmt_requires_parse_analysis() is basically equivalent to the existing function analyze_requires_snapshot(), except that for obscure reasons that function omits ReturnStmt and CallStmt. It is unclear whether those were oversights or intentional. I have not been able to demonstrate a bug from not acquiring a snapshot while analyzing these commands, but at best it seems mighty fragile. It seems safer to acquire a snapshot for parse analysis of these commands too, which allows making stmt_requires_parse_analysis and analyze_requires_snapshot equivalent. In passing this fixes a second bug, which is that ResetPlanCache would exclude ReturnStmts and CallStmts from revalidation. That's surely *not* safe, since they contain parsable expressions. Per bug #18059 from Pavel Kulakov. Back-patch to all supported branches. Discussion: https://postgr.es/m/18059-79c692f036b25346@postgresql.org
src/backend/parser/README Parser ====== This directory does more than tokenize and parse SQL queries. It also creates Query structures for the various complex queries that are passed to the optimizer and then executor. parser.c things start here scan.l break query into tokens scansup.c handle escapes in input strings gram.y parse the tokens and produce a "raw" parse tree analyze.c top level of parse analysis for optimizable queries parse_agg.c handle aggregates, like SUM(col1), AVG(col2), ... parse_clause.c handle clauses like WHERE, ORDER BY, GROUP BY, ... parse_coerce.c handle coercing expressions to different data types parse_collate.c assign collation information in completed expressions parse_cte.c handle Common Table Expressions (WITH clauses) parse_expr.c handle expressions like col, col + 3, x = 3 or x = 4 parse_func.c handle functions, table.column and column identifiers parse_node.c create nodes for various structures parse_oper.c handle operators in expressions parse_param.c handle Params (for the cases used in the core backend) parse_relation.c support routines for tables and column handling parse_target.c handle the result list of the query parse_type.c support routines for data type handling parse_utilcmd.c parse analysis for utility commands (done at execution time) See also src/common/keywords.c, which contains the table of standard keywords and the keyword lookup function. We separated that out because various frontend code wants to use it too.