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

2 files changed, 78 insertions, 2 deletions

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 5da863d85de..88cb97f445f 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -5911,10 +5911,18 @@ plan_create_index_workers(Oid tableOid, Oid indexOid)
 	 * Currently, parallel workers can't access the leader's temporary tables.
 	 * Furthermore, any index predicate or index expressions must be parallel
 	 * safe.
+	 *
+	 * Fetch the list of expressions and predicates directly from the
+	 * catalogs.  Retrieving this information from the relcache would cause
+	 * the expressions and predicates to be flattened, losing properties that
+	 * can be important to check if parallel workers can be used.  For
+	 * example, immutable parallel-unsafe functions, that cannot be used in
+	 * parallel workers, would be changed to Const nodes, that are safe in
+	 * parallel workers.
 	 */
 	if (heap->rd_rel->relpersistence == RELPERSISTENCE_TEMP ||
-		!is_parallel_safe(root, (Node *) RelationGetIndexExpressions(index)) ||
-		!is_parallel_safe(root, (Node *) RelationGetIndexPredicate(index)))
+		!is_parallel_safe(root, (Node *) get_index_expressions(indexOid)) ||
+		!is_parallel_safe(root, (Node *) get_index_predicate(indexOid)))
 	{
 		parallel_workers = 0;
 		goto done;
diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index 5564b53f257..3c232aa960f 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -3511,6 +3511,74 @@ get_index_column_opclass(Oid index_oid, int attno)
 }
 
 /*
+ * 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
  *
  *		Given the index OID, return pg_index.indisreplident.