expression: implement vectorized evaluation for `builtinCastDecimalA…

…sDecimalSig` (pingcap#12102)

expression/builtin_cast_vec.go

@@ -530,11 +530,41 @@ func (b *builtinCastRealAsTimeSig) vecEvalTime(input *chunk.Chunk, result *chunk
 }
 
 func (b *builtinCastDecimalAsDecimalSig) vectorized() bool {
-	return false
+	return true
 }
 
 func (b *builtinCastDecimalAsDecimalSig) vecEvalDecimal(input *chunk.Chunk, result *chunk.Column) error {
-	return errors.Errorf("not implemented")
+	n := input.NumRows()
+	buf, err := b.bufAllocator.get(types.ETDecimal, n)
+	if err != nil {
+		return err
+	}
+	defer b.bufAllocator.put(buf)
+	if err := b.args[0].VecEvalDecimal(b.ctx, input, buf); err != nil {
+		return err
+	}
+
+	result.ResizeDecimal(n, false)
+	result.MergeNulls(buf)
+	oldDecs := buf.Decimals()
+	newDecs := result.Decimals()
+	sc := b.ctx.GetSessionVars().StmtCtx
+	for i := 0; i < n; i++ {
+		if result.IsNull(i) {
+			continue
+		}
+
+		dec := &types.MyDecimal{}
+		if !(b.inUnion && mysql.HasUnsignedFlag(b.tp.Flag) && oldDecs[i].IsNegative()) {
+			*dec = oldDecs[i]
+		}
+		dec, err = types.ProduceDecWithSpecifiedTp(dec, b.tp, sc)
+		if err != nil {
+			return err
+		}
+		newDecs[i] = *dec
+	}
+	return nil
 }
 
 func (b *builtinCastDurationAsTimeSig) vectorized() bool {

expression/builtin_cast_vec_test.go

@@ -79,6 +79,7 @@ var vecBuiltinCastCases = map[string][]vecExprBenchCase{
 			geners: []dataGenerator{
 				&jsonTimeGener{},
 			}},
+		{retEvalType: types.ETDecimal, childrenTypes: []types.EvalType{types.ETDecimal}},
 	},
 }