add support for DEFAULT in insert statements for sequences to the query planner

This will treat DEFAULT as "null" for the purposes of sequences when
running insert statements. This matches with MySQL's documented
behavior and observed resulted.

See for more information:
https://dev.mysql.com/doc/refman/8.0/en/data-type-defaults.html

Signed-off-by: Mark Caudill <mark@sharpspring.com>

go/vt/vtgate/planbuilder/insert.go

@@ -211,13 +211,23 @@ func generateInsertShardedQuery(node *sqlparser.Insert, eins *engine.Insert, val
 
 // modifyForAutoinc modfies the AST and the plan to generate
 // necessary autoinc values. It must be called only if eins.Table.AutoIncrement
-// is set.
+// is set. Bind variable names are generated using baseName.
 func modifyForAutoinc(ins *sqlparser.Insert, eins *engine.Insert) error {
-	pos := findOrAddColumn(ins, eins.Table.AutoIncrement.Column)
-	autoIncValues, err := swapBindVariables(ins.Rows.(sqlparser.Values), pos, ":"+engine.SeqVarName)
-	if err != nil {
-		return err
+	colNum := findOrAddColumn(ins, eins.Table.AutoIncrement.Column)
+	autoIncValues := sqltypes.PlanValue{}
+	for rowNum, row := range ins.Rows.(sqlparser.Values) {
+		// Support the DEFAULT keyword by treating it as null
+		if _, ok := row[colNum].(*sqlparser.Default); ok {
+			row[colNum] = &sqlparser.NullVal{}
+		}
+		pv, err := sqlparser.NewPlanValue(row[colNum])
+		if err != nil {
+			return fmt.Errorf("could not compute value for vindex or auto-inc column: %v", err)
+		}
+		autoIncValues.Values = append(autoIncValues.Values, pv)
+		row[colNum] = sqlparser.NewValArg([]byte(":" + engine.SeqVarName + strconv.Itoa(rowNum)))
 	}
+
 	eins.Generate = &engine.Generate{
 		Keyspace: eins.Table.AutoIncrement.Sequence.Keyspace,
 		Query:    fmt.Sprintf("select next :n values from %s", sqlparser.String(eins.Table.AutoIncrement.Sequence.Name)),
@@ -226,22 +236,6 @@ func modifyForAutoinc(ins *sqlparser.Insert, eins *engine.Insert) error {
 	return nil
 }
 
-// swapBindVariables swaps in bind variable names at the specified
-// column position in the AST values and returns the converted values back.
-// Bind variable names are generated using baseName.
-func swapBindVariables(rows sqlparser.Values, colNum int, baseName string) (sqltypes.PlanValue, error) {
-	pv := sqltypes.PlanValue{}
-	for rowNum, row := range rows {
-		innerpv, err := sqlparser.NewPlanValue(row[colNum])
-		if err != nil {
-			return pv, fmt.Errorf("could not compute value for vindex or auto-inc column: %v", err)
-		}
-		pv.Values = append(pv.Values, innerpv)
-		row[colNum] = sqlparser.NewValArg([]byte(baseName + strconv.Itoa(rowNum)))
-	}
-	return pv, nil
-}
-
 // findOrAddColumn finds the position of a column in the insert. If it's
 // absent it appends it to the with NULL values and returns that position.
 func findOrAddColumn(ins *sqlparser.Insert, col sqlparser.ColIdent) int {

go/vt/vtgate/planbuilder/testdata/dml_cases.txt

@@ -993,6 +993,32 @@
   }
 }
 
+# insert with default seq
+"insert into user(id, nonid) values (default, 2)"
+{
+  "Original": "insert into user(id, nonid) values (default, 2)",
+  "Instructions": {
+    "Opcode": "InsertSharded",
+    "Keyspace": {
+      "Name": "user",
+      "Sharded": true
+    },
+    "Query": "insert into user(id, nonid, Name, Costly) values (:_Id0, 2, :_Name0, :_Costly0)",
+    "Values": [[[":__seq0"]],[[null]],[[null]]],
+    "Table": "user",
+    "Generate": {
+      "Keyspace": {
+        "Name": "main",
+        "Sharded": false
+      },
+      "Query": "select next :n values from seq",
+      "Values": [null]
+    },
+    "Prefix": "insert into user(id, nonid, Name, Costly) values ",
+    "Mid": ["(:_Id0, 2, :_Name0, :_Costly0)"]
+  }
+}
+
 # insert with non vindex bool value
 "insert into user(nonid) values (true)"
 {