From 50b5913a87c9c2d67330d82429f6dda61441d40c Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 6 Mar 2024 17:24:06 +0900
Subject: Fix parallel-safety check of expressions and predicate for index
 builds

As coded, the planner logic that calculates the number of parallel
workers to use for a parallel index build uses expressions and
predicates from the relcache, which are flattened for the planner by
eval_const_expressions().

As reported in the bug, an immutable parallel-unsafe function flattened
in the relcache would become a Const, which would be considered as
parallel-safe, even if the predicate or the expressions including the
function are not safe in parallel workers.  Depending on the expressions
or predicate used, this could cause the parallel build to fail.

Tests are included that check parallel index builds with parallel-unsafe
predicate and expressions.  Two routines are added to lsyscache.h to be
able to retrieve expressions and predicate of an index from its pg_index
data.

Reported-by: Alexander Lakhin
Author: Tender Wang
Reviewed-by: Jian He, Michael Paquier
Discussion: https://postgr.es/m/CAHewXN=UaAaNn9ruHDH3Os8kxLVmtWqbssnf=dZN_s9=evHUFA@mail.gmail.com
Backpatch-through: 12
---
 src/backend/utils/cache/lsyscache.c | 68 +++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

(limited to 'src/backend/utils/cache/lsyscache.c')

diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index 1b7e11b93e0..15a642e5c96 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -3510,6 +3510,74 @@ get_index_column_opclass(Oid index_oid, int attno)
 	return opclass;
 }
 
+/*
+ * get_index_expressions
+ *
+ *		Given the index OID, its a List of its expressions or NIL if none.
+ */
+List *
+get_index_expressions(Oid index_oid)
+{
+	List	   *result;
+	HeapTuple	tuple;
+	Datum		exprDatum;
+	bool		isnull;
+	char	   *exprString;
+
+	tuple = SearchSysCache1(INDEXRELID, ObjectIdGetDatum(index_oid));
+	if (!HeapTupleIsValid(tuple))
+		elog(ERROR, "cache lookup failed for index %u", index_oid);
+
+	exprDatum = SysCacheGetAttr(INDEXRELID, tuple,
+								Anum_pg_index_indexprs, &isnull);
+	if (isnull)
+	{
+		ReleaseSysCache(tuple);
+		return NIL;
+	}
+
+	exprString = TextDatumGetCString(exprDatum);
+	result = (List *) stringToNode(exprString);
+	pfree(exprString);
+	ReleaseSysCache(tuple);
+
+	return result;
+}
+
+/*
+ * get_index_predicate
+ *
+ *		Given the index OID, return a List of its predicate or NIL if none.
+ */
+List *
+get_index_predicate(Oid index_oid)
+{
+	List	   *result;
+	HeapTuple	tuple;
+	Datum		predDatum;
+	bool		isnull;
+	char	   *predString;
+
+	tuple = SearchSysCache1(INDEXRELID, ObjectIdGetDatum(index_oid));
+	if (!HeapTupleIsValid(tuple))
+		elog(ERROR, "cache lookup failed for index %u", index_oid);
+
+	predDatum = SysCacheGetAttr(INDEXRELID, tuple,
+								Anum_pg_index_indpred, &isnull);
+	if (isnull)
+	{
+		ReleaseSysCache(tuple);
+		return NIL;
+	}
+
+	predString = TextDatumGetCString(predDatum);
+	result = (List *) stringToNode(predString);
+	pfree(predString);
+	ReleaseSysCache(tuple);
+
+	return result;
+}
+
 /*
  * get_index_isreplident
  *
-- 
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