plan: fix a bug about prefix index. (pingcap#2445)

plan/refiner.go

@@ -223,6 +223,7 @@ func detachIndexScanConditions(conditions []expression.Expression, indexScan *Ph
 			tableName:    indexScan.Table.Name,
 			idx:          indexScan.Index,
 			columnOffset: curIndex,
+			length:       indexScan.Index.Columns[curIndex].Length,
 		}
 		// First of all, we should extract all of in/eq expressions from rest conditions for every continuous index column.
 		// e.g. For index (a,b,c) and conditions a in (1,2) and b < 1 and c in (3,4), we should only extract column a in (1,2).
@@ -264,7 +265,9 @@ func detachTableScanConditions(conditions []expression.Expression, table *model.
 	var accessConditions, filterConditions []expression.Expression
 	checker := conditionChecker{
 		tableName: table.Name,
-		pkName:    pkName}
+		pkName:    pkName,
+		length:    types.UnspecifiedLength,
+	}
 	for _, cond := range conditions {
 		cond = pushDownNot(cond, false, nil)
 		if !checker.check(cond) {
@@ -307,6 +310,7 @@ type conditionChecker struct {
 	columnOffset  int // the offset of the indexed column to be checked.
 	pkName        model.CIStr
 	shouldReserve bool // check if a access condition should be reserved in filter conditions.
+	length        int
 }
 
 func (c *conditionChecker) check(condition expression.Expression) bool {
@@ -359,10 +363,14 @@ func (c *conditionChecker) checkScalarFunction(scalar *expression.ScalarFunction
 		return c.check(scalar.GetArgs()[0]) && c.check(scalar.GetArgs()[1])
 	case ast.EQ, ast.NE, ast.GE, ast.GT, ast.LE, ast.LT:
 		if _, ok := scalar.GetArgs()[0].(*expression.Constant); ok {
-			return c.checkColumn(scalar.GetArgs()[1])
+			if c.checkColumn(scalar.GetArgs()[1]) {
+				return scalar.FuncName.L != ast.NE || c.length == types.UnspecifiedLength
+			}
 		}
 		if _, ok := scalar.GetArgs()[1].(*expression.Constant); ok {
-			return c.checkColumn(scalar.GetArgs()[0])
+			if c.checkColumn(scalar.GetArgs()[0]) {
+				return scalar.FuncName.L != ast.NE || c.length == types.UnspecifiedLength
+			}
 		}
 	case ast.IsNull, ast.IsTruth, ast.IsFalsity:
 		return c.checkColumn(scalar.GetArgs()[0])