sql/sem: remove EvalContext.getTmpDec

`apd.Decimal` can now be entirely stack allocated during arithmetic, so
there's no longer any need for this.

With cockroachdb/apd#104, this does not introduce
any new heap allocations:
```
➜ (cd pkg/sql/sem/tree && goescape . | grep moved | wc -l)
     328
```

pkg/sql/sem/tree/cast.go

@@ -2755,12 +2755,12 @@ func performCastWithoutPrecisionTruncation(
 		case *DTime:
 			return NewDInterval(duration.MakeDuration(int64(*v)*1000, 0, 0), itm), nil
 		case *DDecimal:
-			d := ctx.getTmpDec()
+			var d apd.Decimal
 			dnanos := v.Decimal
 			dnanos.Exponent += 9
 			// We need HighPrecisionCtx because duration values can contain
 			// upward of 35 decimal digits and DecimalCtx only provides 25.
-			_, err := HighPrecisionCtx.Quantize(d, &dnanos, 0)
+			_, err := HighPrecisionCtx.Quantize(&d, &dnanos, 0)
 			if err != nil {
 				return nil, err
 			}
@@ -2921,8 +2921,8 @@ func performIntToOidCast(ctx *EvalContext, t *types.T, v DInt) (Datum, error) {
 }
 
 func roundDecimalToInt(ctx *EvalContext, d *apd.Decimal) (int64, error) {
-	newD := ctx.getTmpDec()
-	if _, err := DecimalCtx.RoundToIntegralValue(newD, d); err != nil {
+	var newD apd.Decimal
+	if _, err := DecimalCtx.RoundToIntegralValue(&newD, d); err != nil {
 		return 0, err
 	}
 	i, err := newD.Int64()

pkg/sql/sem/tree/datum.go

@@ -1057,10 +1057,10 @@ func (d *DDecimal) CompareError(ctx *EvalContext, other Datum) (int, error) {
 		// NULL is less than any non-NULL value.
 		return 1, nil
 	}
-	v := ctx.getTmpDec()
+	var v apd.Decimal
 	switch t := UnwrapDatum(ctx, other).(type) {
 	case *DDecimal:
-		v = &t.Decimal
+		v.Set(&t.Decimal)
 	case *DInt:
 		v.SetInt64(int64(*t))
 	case *DFloat:
@@ -1070,7 +1070,7 @@ func (d *DDecimal) CompareError(ctx *EvalContext, other Datum) (int, error) {
 	default:
 		return 0, makeUnsupportedComparisonMessage(d, other)
 	}
-	res := CompareDecimals(&d.Decimal, v)
+	res := CompareDecimals(&d.Decimal, &v)
 	return res, nil
 }
 

pkg/sql/sem/tree/eval.go

@@ -1428,10 +1428,11 @@ var BinOps = map[BinaryOperatorSymbol]binOpOverload{
 				if rInt == 0 {
 					return nil, ErrDivByZero
 				}
-				div := ctx.getTmpDec().SetInt64(int64(rInt))
+				var div apd.Decimal
+				div.SetInt64(int64(rInt))
 				dd := &DDecimal{}
 				dd.SetInt64(int64(MustBeDInt(left)))
-				_, err := DecimalCtx.Quo(&dd.Decimal, &dd.Decimal, div)
+				_, err := DecimalCtx.Quo(&dd.Decimal, &dd.Decimal, &div)
 				return dd, err
 			},
 			Volatility: VolatilityImmutable,
@@ -3930,10 +3931,6 @@ func (ctx *EvalContext) Ctx() context.Context {
 	return ctx.Context
 }
 
-func (ctx *EvalContext) getTmpDec() *apd.Decimal {
-	return &ctx.tmpDec
-}
-
 // Eval implements the TypedExpr interface.
 func (expr *AndExpr) Eval(ctx *EvalContext) (Datum, error) {
 	left, err := expr.Left.(TypedExpr).Eval(ctx)