colbuilder: pre-evaluate casts of constants in projection exprs

During the planning of the projection expressions we have certain
optimized operators if either left or right expressions are constants.
Previously, we would simply check whether expression is a datum to see
whether it is a constant. Another simple case that this commit adds is
if we have a cast expression of a datum - this commit adds
pre-evaluation of such expressions allowing us to plan optimized
operators more often. Notably, this handles a case of a cast from NULL
which the optimizer creates in order to propagate the type of NULL.

Release note: None

pkg/sql/colexec/colbuilder/execplan.go

@@ -2101,6 +2101,19 @@ func checkSupportedBinaryExpr(left, right tree.TypedExpr, outputType *types.T) e
 	return nil
 }
 
+// preEvaluateConstCast checks whether t is a cast expression from a constant
+// datum and evaluates the cast if so.
+func preEvaluateConstCast(evalCtx *tree.EvalContext, t tree.TypedExpr) (tree.TypedExpr, error) {
+	cast, ok := t.(*tree.CastExpr)
+	if !ok {
+		return t, nil
+	}
+	if _, isConst := cast.Expr.(tree.Datum); !isConst {
+		return t, nil
+	}
+	return cast.Eval(evalCtx)
+}
+
 func planProjectionExpr(
 	ctx context.Context,
 	evalCtx *tree.EvalContext,
@@ -2117,6 +2130,18 @@ func planProjectionExpr(
 	if err := checkSupportedProjectionExpr(left, right); err != nil {
 		return nil, resultIdx, typs, err
 	}
+	// Normally, the optimizer folds all constants; however, for nulls it
+	// creates a cast expression from tree.DNull to the desired type in order to
+	// propagate the type of the null. Pre-evaluate constant casts so that the
+	// optimized projection operators below could be planned.
+	left, err = preEvaluateConstCast(evalCtx, left)
+	if err != nil {
+		return nil, resultIdx, typs, err
+	}
+	right, err = preEvaluateConstCast(evalCtx, right)
+	if err != nil {
+		return nil, resultIdx, typs, err
+	}
 	allocator := colmem.NewAllocator(ctx, acc, factory)
 	resultIdx = -1
 	// There are 3 cases. Either the left is constant, the right is constant,

pkg/sql/logictest/testdata/logic_test/vectorize_local

@@ -347,3 +347,19 @@ EXPLAIN (VEC) SELECT concat_agg(_bytes), concat_agg(_string) FROM bytes_string
   └ *colexec.orderedAggregator
     └ *colexecbase.distinctChainOps
       └ *colfetcher.ColBatchScan
+
+statement ok
+CREATE TABLE t63792 (c INT)
+
+# Check that casts of constants are pre-evaluated (which allows us to use
+# colexec.isNullProjOp instead of colexecproj.defaultCmpProjOp).
+query T
+EXPLAIN (VEC) SELECT c = c FROM t63792
+----
+│
+└ Node 1
+  └ *colexec.orProjOp
+    ├ *colfetcher.ColBatchScan
+    ├ *colexec.isNullProjOp
+    └ *colexecbase.castOpNullAny
+      └ *colexecbase.constNullOp