Function DatumPrev

pkg/sql/sem/tree/datum.go:6583–6635

DatumPrev returns a datum that is "previous" to the given one. For many types it just delegates to Datum.Prev, but for some types that don't have an implementation of that function this method makes the best effort to come up with a reasonable previous datum that is smaller than the given one. The

(
	ctx context.Context, datum Datum, cmpCtx CompareContext, collationEnv *CollationEnvironment,
)

// The return value is undefined if Datum.IsMin returns true or if the value is
// NaN or an infinity (for floats and decimals).
func DatumPrev(
	ctx context.Context, datum Datum, cmpCtx CompareContext, collationEnv *CollationEnvironment,
) (Datum, bool) {
	datum = UnwrapDOidWrapper(datum)
	prevString := func(s string) (string, bool) {
		// In order to obtain a previous string we subtract 1 from the last
		// non-zero byte.
		b := []byte(s)
		lastNonZeroByteIdx := len(b) - 1
		for ; lastNonZeroByteIdx >= 0 && b[lastNonZeroByteIdx] == 0; lastNonZeroByteIdx-- {
		}
		if lastNonZeroByteIdx < 0 {
			return "", false
		}
		b[lastNonZeroByteIdx]--
		return string(b), true
	}
	switch d := datum.(type) {
	case *DDecimal:
		var prev DDecimal
		var sub apd.Decimal
		_, err := sub.SetFloat64(1e-6)
		if err != nil {
			return nil, false
		}
		_, err = ExactCtx.Sub(&prev.Decimal, &d.Decimal, &sub)
		if err != nil {
			return nil, false
		}
		return &prev, true
	case *DString:
		prev, ok := prevString(string(*d))
		if !ok {
			return nil, false
		}
		return NewDString(prev), true
	case *DBytes:
		prev, ok := prevString(string(*d))
		if !ok {
			return nil, false
		}
		return NewDBytes(DBytes(prev)), true
	case *DInterval:
		// Subtract 1ms.
		prev := d.Sub(duration.MakeDuration(1000000 /* nanos */, 0 /* days */, 0 /* months */))
		return NewDInterval(prev, types.DefaultIntervalTypeMetadata), true
	default:
		// TODO(yuzefovich): consider adding support for other datums that don't
		// have Datum.Prev implementation (DCollatedString, DBitArray,
		// DGeography, DGeometry, DBox2D, DJSON, DArray).
		return datum.Prev(ctx, cmpCtx)
	}
}

// DatumNext returns a datum that is "next" to the given one. For many types it
// just delegates to Datum.Next, but for some types that don't have an