Made LOAD DATA handle NULL values and defaults correctly

Author: timsehn

enginetest/queries/load_queries.go

@@ -22,6 +22,19 @@ import (
 )
 
 var LoadDataScripts = []ScriptTest{
+	{
+		Name: "LOAD DATA applies column defaults when \\N provided",
+		SetUpScript: []string{
+			"create table t (pk int primary key, c1 int default 1, c2 int)",
+			"LOAD DATA INFILE './testdata/load_defaults_null.csv' INTO TABLE t FIELDS TERMINATED BY ',' LINES TERMINATED BY '\n'",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query:    "select * from t",
+				Expected: []sql.Row{{1, 1, 1}},
+			},
+		},
+	},
 	{
 		Name: "Basic load data with enclosed values.",
 		SetUpScript: []string{

enginetest/testdata/load_defaults_null.csv

@@ -0,0 +1 @@
+1,\N,1

sql/rowexec/ddl_iters.go

@@ -206,8 +206,20 @@ func (l *loadDataIter) parseFields(ctx *sql.Context, line string) ([]sql.Express
 					exprs[exprIdx] = expression.NewLiteral(nil, types.Null)
 				}
 			}
-		case "NULL":
-			exprs[exprIdx] = expression.NewLiteral(nil, types.Null)
+        case "NULL":
+            // For MySQL LOAD DATA semantics, \N (mapped to NULL here) should use the column default
+            // if one exists; otherwise insert NULL.
+            destIdx := l.fieldToColMap[fieldIdx]
+            if destIdx >= 0 {
+                destCol := l.destSch[destIdx]
+                if destCol.Default != nil {
+                    exprs[exprIdx] = destCol.Default
+                } else {
+                    exprs[exprIdx] = expression.NewLiteral(nil, types.Null)
+                }
+            } else {
+                exprs[exprIdx] = expression.NewLiteral(nil, types.Null)
+            }
 		default:
 			exprs[exprIdx] = expression.NewLiteral(field, types.LongText)
 		}