statistics: reduce small obj alloc for PreCalculateScalar (#52623) (#…

…52652)

close #52343

pkg/statistics/scalar.go

@@ -101,23 +101,26 @@ func (hg *Histogram) PreCalculateScalar() {
 	}
 	switch hg.GetLower(0).Kind() {
 	case types.KindMysqlDecimal, types.KindMysqlTime:
+		var lower, upper types.Datum
 		hg.Scalars = make([]scalar, l)
 		for i := 0; i < l; i++ {
-			hg.Scalars[i] = scalar{
-				lower: convertDatumToScalar(hg.GetLower(i), 0),
-				upper: convertDatumToScalar(hg.GetUpper(i), 0),
-			}
+			// It's read-only, so we don't need to allocate new datum each time.
+			hg.LowerToDatum(i, &lower)
+			hg.UpperToDatum(i, &upper)
+			hg.Scalars[i].lower = convertDatumToScalar(&lower, 0)
+			hg.Scalars[i].upper = convertDatumToScalar(&upper, 0)
 		}
 	case types.KindBytes, types.KindString:
+		var lower, upper types.Datum
 		hg.Scalars = make([]scalar, l)
 		for i := 0; i < l; i++ {
-			lower, upper := hg.GetLower(i), hg.GetUpper(i)
+			// It's read-only, so we don't need to allocate new datum each time.
+			hg.LowerToDatum(i, &lower)
+			hg.UpperToDatum(i, &upper)
 			common := commonPrefixLength(lower.GetBytes(), upper.GetBytes())
-			hg.Scalars[i] = scalar{
-				commonPfxLen: common,
-				lower:        convertDatumToScalar(lower, common),
-				upper:        convertDatumToScalar(upper, common),
-			}
+			hg.Scalars[i].commonPfxLen = common
+			hg.Scalars[i].lower = convertDatumToScalar(&lower, common)
+			hg.Scalars[i].upper = convertDatumToScalar(&upper, common)
 		}
 	}
 }