small optimisation that makes big difference

Before optimisation:
BenchmarkPlanner/large_cases.txt-v4-16  70786    167749 ns/op   74315 B/op    1618 allocs/op
BenchmarkPlanner/large_cases.txt-v4-16  70998    168734 ns/op   74303 B/op    1618 allocs/op

After:
BenchmarkPlanner/large_cases.txt-v4-16  76375    156411 ns/op   67752 B/op    1376 allocs/op
BenchmarkPlanner/large_cases.txt-v4-16  76207    157553 ns/op   67816 B/op    1376 allocs/op

Signed-off-by: Andres Taylor <andres@planetscale.com>

go/vt/vtgate/planbuilder/querygraph_test.go

@@ -123,7 +123,6 @@ func TestQueryGraph(t *testing.T) {
 			require.NoError(t, err)
 			qgraph, err := createQGFromSelect(tree.(*sqlparser.Select), semTable)
 			require.NoError(t, err)
-			fmt.Println(qgraph.testString())
 			assert.Equal(t, tc.output, qgraph.testString())
 			utils.MustMatch(t, tc.output, qgraph.testString(), "incorrect query graph")
 		})

go/vt/vtgate/planbuilder/route_planning_test.go

@@ -99,7 +99,7 @@ func TestMergeJoins(t *testing.T) {
 	}}
 	for i, tc := range tests {
 		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
-			result := tryMerge(tc.l, tc.r, tc.predicates, &semantics.SemTable{})
+			result := tryMerge(tc.l, tc.r, tc.predicates, semantics.NewSemTable())
 			assert.Equal(t, tc.expected, result)
 		})
 	}

go/vt/vtgate/semantics/semantic_state.go

@@ -42,6 +42,11 @@ type (
 	}
 )
 
+// NewSemTable creates a new empty SemTable
+func NewSemTable() *SemTable {
+	return &SemTable{exprDependencies: map[sqlparser.Expr]TableSet{}}
+}
+
 // TableSetFor returns the bitmask for this particular tableshoe
 func (st *SemTable) TableSetFor(t table) TableSet {
 	for idx, t2 := range st.Tables {
@@ -54,7 +59,10 @@ func (st *SemTable) TableSetFor(t table) TableSet {
 
 // Dependencies return the table dependencies of the expression.
 func (st *SemTable) Dependencies(expr sqlparser.Expr) TableSet {
-	var deps TableSet
+	deps, found := st.exprDependencies[expr]
+	if found {
+		return deps
+	}
 
 	_ = sqlparser.Walk(func(node sqlparser.SQLNode) (kontinue bool, err error) {
 		colName, ok := node.(*sqlparser.ColName)
@@ -65,6 +73,8 @@ func (st *SemTable) Dependencies(expr sqlparser.Expr) TableSet {
 		return true, nil
 	}, expr)
 
+	st.exprDependencies[expr] = deps
+
 	return deps
 }