More code review for get_qual_for_list().

Avoid trashing the input PartitionBoundSpec; while that might be safe for
current callers, it's certainly trouble waiting to happen.  In the same
vein, make sure that all of the result data structure is freshly palloc'd,
rather than some of it being pointers into the input data structures
(which we don't know the lifespans of).

Simplify the logic for tacking on IS NULL or IS NOT NULL conditions some
more; commit 85c2b9a1 left a lot on the table there.  And rearrange the
construction of the nodes into (what seems to me) a more logical order.

In passing, make sure that get_qual_for_range() also returns a freshly
palloc'd structure, since there's no value in having that guarantee for
only one kind of partitioning.  And improve some comments there.

Jeevan Ladhe, with further tweaking by me

Discussion: https://postgr.es/m/CAOgcT0MAcYoMs93W80iTUf_dP36=1mZQzeUk+nnwY_-qWDrCfw@mail.gmail.com

src/backend/catalog/partition.c

@@ -1296,23 +1296,22 @@ make_partition_op_expr(PartitionKey key, int keynum,
 /*
  * get_qual_for_list
  *
- * Returns a list of expressions to use as a list partition's constraint.
+ * Returns an implicit-AND list of expressions to use as a list partition's
+ * constraint, given the partition key and bound structures.
  */
 static List *
 get_qual_for_list(PartitionKey key, PartitionBoundSpec *spec)
 {
 	List	   *result;
+	Expr	   *keyCol;
 	ArrayExpr  *arr;
 	Expr	   *opexpr;
-	ListCell   *cell,
-			   *prev,
-			   *next;
-	Expr	   *keyCol;
+	NullTest   *nulltest;
+	ListCell   *cell;
+	List	   *arrelems = NIL;
 	bool		list_has_null = false;
-	NullTest   *nulltest1 = NULL,
-			   *nulltest2 = NULL;
 
-	/* Left operand is either a simple Var or arbitrary expression */
+	/* Construct Var or expression representing the partition column */
 	if (key->partattrs[0] != 0)
 		keyCol = (Expr *) makeVar(1,
 								  key->partattrs[0],
@@ -1323,59 +1322,25 @@ get_qual_for_list(PartitionKey key, PartitionBoundSpec *spec)
 	else
 		keyCol = (Expr *) copyObject(linitial(key->partexprs));
 
-	/*
-	 * We must remove any NULL value in the list; we handle it separately
-	 * below.
-	 */
-	prev = NULL;
-	for (cell = list_head(spec->listdatums); cell; cell = next)
+	/* Create list of Consts for the allowed values, excluding any nulls */
+	foreach(cell, spec->listdatums)
 	{
 		Const	   *val = castNode(Const, lfirst(cell));
 
-		next = lnext(cell);
-
 		if (val->constisnull)
-		{
 			list_has_null = true;
-			spec->listdatums = list_delete_cell(spec->listdatums,
-												cell, prev);
-		}
 		else
-			prev = cell;
+			arrelems = lappend(arrelems, copyObject(val));
 	}
 
-	if (!list_has_null)
-	{
-		/*
-		 * Gin up a col IS NOT NULL test that will be AND'd with other
-		 * expressions
-		 */
-		nulltest1 = makeNode(NullTest);
-		nulltest1->arg = keyCol;
-		nulltest1->nulltesttype = IS_NOT_NULL;
-		nulltest1->argisrow = false;
-		nulltest1->location = -1;
-	}
-	else
-	{
-		/*
-		 * Gin up a col IS NULL test that will be OR'd with other expressions
-		 */
-		nulltest2 = makeNode(NullTest);
-		nulltest2->arg = keyCol;
-		nulltest2->nulltesttype = IS_NULL;
-		nulltest2->argisrow = false;
-		nulltest2->location = -1;
-	}
-
-	/* Right operand is an ArrayExpr containing this partition's values */
+	/* Construct an ArrayExpr for the non-null partition values */
 	arr = makeNode(ArrayExpr);
 	arr->array_typeid = !type_is_array(key->parttypid[0])
 		? get_array_type(key->parttypid[0])
 		: key->parttypid[0];
 	arr->array_collid = key->parttypcoll[0];
 	arr->element_typeid = key->parttypid[0];
-	arr->elements = spec->listdatums;
+	arr->elements = arrelems;
 	arr->multidims = false;
 	arr->location = -1;
 
@@ -1383,14 +1348,36 @@ get_qual_for_list(PartitionKey key, PartitionBoundSpec *spec)
 	opexpr = make_partition_op_expr(key, 0, BTEqualStrategyNumber,
 									keyCol, (Expr *) arr);
 
-	if (nulltest1)
-		result = list_make2(nulltest1, opexpr);
+	if (!list_has_null)
+	{
+		/*
+		 * Gin up a "col IS NOT NULL" test that will be AND'd with the main
+		 * expression.  This might seem redundant, but the partition routing
+		 * machinery needs it.
+		 */
+		nulltest = makeNode(NullTest);
+		nulltest->arg = keyCol;
+		nulltest->nulltesttype = IS_NOT_NULL;
+		nulltest->argisrow = false;
+		nulltest->location = -1;
+
+		result = list_make2(nulltest, opexpr);
+	}
 	else
 	{
+		/*
+		 * Gin up a "col IS NULL" test that will be OR'd with the main
+		 * expression.
+		 */
 		Expr	   *or;
 
-		Assert(nulltest2 != NULL);
-		or = makeBoolExpr(OR_EXPR, list_make2(nulltest2, opexpr), -1);
+		nulltest = makeNode(NullTest);
+		nulltest->arg = keyCol;
+		nulltest->nulltesttype = IS_NULL;
+		nulltest->argisrow = false;
+		nulltest->location = -1;
+
+		or = makeBoolExpr(OR_EXPR, list_make2(nulltest, opexpr), -1);
 		result = list_make1(or);
 	}
 
@@ -1401,8 +1388,16 @@ get_qual_for_list(PartitionKey key, PartitionBoundSpec *spec)
  * get_range_key_properties
  *		Returns range partition key information for a given column
  *
- * On return, *partexprs_item points to the cell containing the next
- * expression in the key->partexprs list, or NULL.
+ * This is a subroutine for get_qual_for_range, and its API is pretty
+ * specialized to that caller.
+ *
+ * Constructs an Expr for the key column (returned in *keyCol) and Consts
+ * for the lower and upper range limits (returned in *lower_val and
+ * *upper_val).  For UNBOUNDED limits, NULL is returned instead of a Const.
+ * All of these structures are freshly palloc'd.
+ *
+ * *partexprs_item points to the cell containing the next expression in
+ * the key->partexprs list, or NULL.  It may be advanced upon return.
  */
 static void
 get_range_key_properties(PartitionKey key, int keynum,
@@ -1412,7 +1407,7 @@ get_range_key_properties(PartitionKey key, int keynum,
 						 Expr **keyCol,
 						 Const **lower_val, Const **upper_val)
 {
-	/* Partition key expression for this column */
+	/* Get partition key expression for this column */
 	if (key->partattrs[keynum] != 0)
 	{
 		*keyCol = (Expr *) makeVar(1,
@@ -1424,17 +1419,20 @@ get_range_key_properties(PartitionKey key, int keynum,
 	}
 	else
 	{
+		if (*partexprs_item == NULL)
+			elog(ERROR, "wrong number of partition key expressions");
 		*keyCol = copyObject(lfirst(*partexprs_item));
 		*partexprs_item = lnext(*partexprs_item);
 	}
 
+	/* Get appropriate Const nodes for the bounds */
 	if (!ldatum->infinite)
-		*lower_val = castNode(Const, ldatum->value);
+		*lower_val = castNode(Const, copyObject(ldatum->value));
 	else
 		*lower_val = NULL;
 
 	if (!udatum->infinite)
-		*upper_val = castNode(Const, udatum->value);
+		*upper_val = castNode(Const, copyObject(udatum->value));
 	else
 		*upper_val = NULL;
 }
@@ -1442,9 +1440,8 @@ get_range_key_properties(PartitionKey key, int keynum,
 /*
  * get_qual_for_range
  *
- * Get a list of expressions to use as a range partition's constraint.
- * If there are multiple expressions, they are to be considered implicitly
- * ANDed.
+ * Returns an implicit-AND list of expressions to use as a range partition's
+ * constraint, given the partition key and bound structures.
  *
  * For a multi-column range partition key, say (a, b, c), with (al, bl, cl)
  * as the lower bound tuple and (au, bu, cu) as the upper bound tuple, we
@@ -1480,9 +1477,8 @@ get_range_key_properties(PartitionKey key, int keynum,
  * does not really have a constraint, except the IS NOT NULL constraint for
  * partition keys.
  *
- * If we end up with an empty result list, we append return a single-member
- * list containing a constant-true expression in that case, because callers
- * expect a non-empty list.
+ * If we end up with an empty result list, we return a single-member list
+ * containing a constant TRUE, because callers expect a non-empty list.
  */
 static List *
 get_qual_for_range(PartitionKey key, PartitionBoundSpec *spec)
@@ -1573,7 +1569,7 @@ get_qual_for_range(PartitionKey key, PartitionBoundSpec *spec)
 		/*
 		 * Since get_range_key_properties() modifies partexprs_item, and we
 		 * might need to start over from the previous expression in the later
-		 * part of this functiom, save away the current value.
+		 * part of this function, save away the current value.
 		 */
 		partexprs_item_saved = partexprs_item;
 
@@ -1638,6 +1634,7 @@ get_qual_for_range(PartitionKey key, PartitionBoundSpec *spec)
 		List	   *lower_or_arm_args = NIL,
 				   *upper_or_arm_args = NIL;
 
+		/* Restart scan of columns from the i'th one */
 		j = i;
 		partexprs_item = partexprs_item_saved;
 
@@ -1702,7 +1699,6 @@ get_qual_for_range(PartitionKey key, PartitionBoundSpec *spec)
 																   strategy,
 																   keyCol,
 														(Expr *) upper_val));
-
 			}
 
 			/*
@@ -1759,7 +1755,7 @@ get_qual_for_range(PartitionKey key, PartitionBoundSpec *spec)
 						 : linitial(upper_or_arms));
 
 	/* As noted above, caller expects the list to be non-empty. */
-	if (result == NULL)
+	if (result == NIL)
 		result = list_make1(makeBoolConst(true, false));
 
 	return result;