MB-53028 Do not generate index filters if _EMPTY_SPAN is used

As a special case, if an index scan is using _EMPTY_SPAN,
then nothing will be returned from the index scan, and
in such cases there is no need to generate index filters
as well as other optimizations based on index keys such
as early order and bloom filters.

Change-Id: Ib9dc24532b1432a406fd55aac1d31cdd234e0d3e
Reviewed-on: https://review.couchbase.org/c/query/+/177756
Reviewed-by: Sitaram Vemulapalli <sitaram.vemulapalli@couchbase.com>
Tested-by: Bingjie Miao <bingjie.miao@couchbase.com>

planner/build_scan_secondary.go

@@ -1027,6 +1027,11 @@ func (this *builder) eqJoinFilter(fl *base.Filter, alias string) (bool, expressi
 func (this *builder) getIndexFilters(entry *indexEntry, node *algebra.KeyspaceTerm,
 	baseKeyspace *base.BaseKeyspace, id expression.Expression) (err error) {
 
+	// special case, if the span is an empty span, no need to proceed
+	if isSpecialSpan(entry.spans, plan.RANGE_EMPTY_SPAN) {
+		return nil
+	}
+
 	alias := node.Alias()
 	useCBO := this.useCBO && this.keyspaceUseCBO(alias)
 	advisorValidate := this.advisorValidate()

planner/spans_term.go

@@ -554,6 +554,19 @@ func setSpecialSpan(rg *plan.Range2) {
 	}
 }
 
+func isSpecialSpan(sspans SargSpans, flag uint32) bool {
+	if tspans, ok := sspans.(*TermSpans); ok {
+		spans := tspans.spans
+		if len(spans) == 1 {
+			rgs := spans[0].Ranges
+			if len(rgs) == 1 {
+				return (rgs[0].Flags & flag) != 0
+			}
+		}
+	}
+	return false
+}
+
 func ConvertSpans2ToSpan(spans2 plan.Spans2, total int) (plan.Spans, bool) {
 	exact := true
 	spans := make(plan.Spans, 0, len(spans2))