colfetcher: find needed columns upfront

Previously, the two users of the `cFetcher` (`ColBatchScan` and
`ColIndexJoin`) had different ways of figuring out which columns are
actually needed to be fetched (that information is needed to not decode
parts of the KV pairs unnecessarily). In one case we explicitly
propagated the information through the spec, and in another we did some
pre-processing to find the information.

This overall was more complicated than necessary (because we had complex
code path for pruning "unaccessible" columns from the secondary index
while keeping track of the ordinal mapping) as well as suboptimal (in
some cases `TableReaderSpec.NeededColumns` contained ordinals of the
columns that weren't actually needed by the post-processing stage), so
this commit unifies both ways into one addressing both of these
shortcomings.

Now, in order to find the set of needed columns, we examine the
post-processing spec all the time, but we do it in an efficient manner:
- we make sure to deserialize render expressions early
- we use a `tree.Visitor` rather than a heavy-weight `ExprHelper`.

As a result, we can remove `TableReaderSpec.NeededColumns` field as well
as not decode some columns in a handful of cases.

Release note: None

pkg/sql/colexec/colbuilder/execplan_test.go

@@ -84,9 +84,8 @@ func TestNewColOperatorExpectedTypeSchema(t *testing.T) {
 
 	desc := catalogkv.TestingGetTableDescriptor(kvDB, keys.SystemSQLCodec, "test", "t")
 	tr := execinfrapb.TableReaderSpec{
-		Table:         *desc.TableDesc(),
-		Spans:         make([]roachpb.Span, 1),
-		NeededColumns: []uint32{0},
+		Table: *desc.TableDesc(),
+		Spans: make([]roachpb.Span, 1),
 	}
 	var err error
 	tr.Spans[0].Key, err = randgen.TestingMakePrimaryIndexKey(desc, 0)

pkg/sql/colexec/colexecargs/BUILD.bazel

@@ -16,7 +16,6 @@ go_library(
         "//pkg/sql/colexecop",
         "//pkg/sql/execinfra",
         "//pkg/sql/execinfrapb",
-        "//pkg/sql/parser",
         "//pkg/sql/sem/tree",
         "//pkg/sql/types",
         "//pkg/util/mon",

pkg/sql/colexec/colexecargs/expr.go

@@ -14,7 +14,6 @@ import (
 	"sync"
 
 	"github.com/cockroachdb/cockroach/pkg/sql/execinfrapb"
-	"github.com/cockroachdb/cockroach/pkg/sql/parser"
 	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
 	"github.com/cockroachdb/cockroach/pkg/sql/types"
 )
@@ -51,55 +50,3 @@ func (h *ExprHelper) ProcessExpr(
 	tempVars := tree.MakeIndexedVarHelper(&h.helper, len(typs))
 	return execinfrapb.DeserializeExpr(expr.Expr, h.SemaCtx, evalCtx, &tempVars)
 }
-
-// Remove unused warning.
-var _ = findIVarsInRange
-
-// findIVarsInRange searches Expr for presence of tree.IndexedVars with indices
-// in range [start, end). It returns a slice containing all such indices.
-func findIVarsInRange(expr execinfrapb.Expression, start int, end int) ([]uint32, error) {
-	res := make([]uint32, 0)
-	if start >= end {
-		return res, nil
-	}
-	var exprToWalk tree.Expr
-	if expr.LocalExpr != nil {
-		exprToWalk = expr.LocalExpr
-	} else {
-		e, err := parser.ParseExpr(expr.Expr)
-		if err != nil {
-			return nil, err
-		}
-		exprToWalk = e
-	}
-	visitor := ivarExpressionVisitor{ivarSeen: make([]bool, end)}
-	_, _ = tree.WalkExpr(visitor, exprToWalk)
-	for i := start; i < end; i++ {
-		if visitor.ivarSeen[i] {
-			res = append(res, uint32(i))
-		}
-	}
-	return res, nil
-}
-
-type ivarExpressionVisitor struct {
-	ivarSeen []bool
-}
-
-var _ tree.Visitor = &ivarExpressionVisitor{}
-
-// VisitPre is a part of tree.Visitor interface.
-func (i ivarExpressionVisitor) VisitPre(expr tree.Expr) (bool, tree.Expr) {
-	switch e := expr.(type) {
-	case *tree.IndexedVar:
-		if e.Idx < len(i.ivarSeen) {
-			i.ivarSeen[e.Idx] = true
-		}
-		return false, expr
-	default:
-		return true, expr
-	}
-}
-
-// VisitPost is a part of tree.Visitor interface.
-func (i ivarExpressionVisitor) VisitPost(expr tree.Expr) tree.Expr { return expr }

pkg/sql/colfetcher/BUILD.bazel

@@ -26,6 +26,7 @@ go_library(
         "//pkg/sql/colencoding",
         "//pkg/sql/colexec/colexecargs",
         "//pkg/sql/colexec/colexecspan",
+        "//pkg/sql/colexec/colexecutils",
         "//pkg/sql/colexecerror",
         "//pkg/sql/colexecop",
         "//pkg/sql/colmem",