colexec: fix projections with constant NULL values

This commit fixes a bug in the vectorized engine that caused internal
errors when projections operated on constant NULL values. The
proj_const_right_ops operators do not correctly handle NULL inputs. So,
we avoid these code paths when operator does not operate on NULL values
and the LHS or RHS of an operator is NULL by simply projecting NULLs.
This is similar to the change made in cockroachdb#128123 and addresses a TODO added
in that PR.

This code path is unlikely to hit (maybe impossible) without generic
query plans. In an optimized cusotm plan, the optimizer will fold
expressions with `NULL` values at optimization time, and the execution
engine will never see expressions of this form.

Fixes cockroachdb#152771

Release note (bug fix): A bug has been fixed that could cause internal
errors for queries using generic query plans with `NULL` placeholder
values.

Author: mgartner

pkg/sql/colexec/colbuilder/execplan.go

@@ -2828,11 +2828,16 @@ func planProjectionExpr(
 		resultIdx = len(typs)
 		// The projection result will be outputted to a new column which is
 		// appended to the input batch.
-		// TODO(#127814): We may need to handle the case when the left is DNull.
-		op, err = colexecprojconst.GetProjectionLConstOperator(
-			allocator, typs, left.ResolvedType(), outputType, projOp, input,
-			rightIdx, lConstArg, resultIdx, evalCtx, binOp, cmpExpr, calledOnNullInput,
-		)
+		if !calledOnNullInput && (left == tree.DNull || right == tree.DNull) {
+			// If the left or right is NULL and the operator is not called on
+			// NULL, simply project NULL.
+			op = colexecbase.NewConstNullOp(allocator, outputType, input, resultIdx)
+		} else {
+			op, err = colexecprojconst.GetProjectionLConstOperator(
+				allocator, typs, left.ResolvedType(), outputType, projOp, input,
+				rightIdx, lConstArg, resultIdx, evalCtx, binOp, cmpExpr, calledOnNullInput,
+			)
+		}
 	} else {
 		var leftIdx int
 		input, leftIdx, typs, err = planProjectionOperators(

pkg/sql/logictest/testdata/logic_test/vectorize

@@ -1348,3 +1348,26 @@ statement ok
 RESET vectorize
 
 subtest end
+
+# Regression test for #152771. Gracefully plan operators that cannot handle NULL
+# inputs.
+statement ok
+CREATE TABLE t152771 (i INT)
+
+statement ok
+INSERT INTO t152771 VALUES (0)
+
+statement ok
+SET plan_cache_mode = force_generic_plan
+
+statement ok
+PREPARE p(INT, INT, INT, INT) AS UPDATE t152771 SET i=($1-$2)+($3-$4)
+
+statement ok
+EXECUTE p(NULL, 3, 33, NULL)
+
+statement ok
+DEALLOCATE p
+
+statement ok
+RESET plan_cache_mode