// mergeAndSortSpans is used to merge a set of potentially overlapping spans
// into a sorted set of non-overlapping spans.
func mergeAndSortSpans(s sqlbase.Spans) sqlbase.Spans {
// This is the classic 1D geometry problem of merging overlapping segments
// on the X axis. It can be solved using a scan algorithm: we go through all
// segment starting and ending points in X order (as "events") and keep
// track of how many open segments we have at each point.
events := make(spanEvents, 2*len(s))
for i := range s {
events[2*i] = spanEvent{start: true, key: s[i].Start}
events[2*i+1] = spanEvent{start: false, key: s[i].End}
if s[i].Start.Compare(s[i].End) >= 0 {
panic(fmt.Sprintf("invalid input span %s", sqlbase.PrettySpan(s[i], 0)))
}
}
sort.Sort(events)
openSpans := 0
s = s[:0]
for _, e := range events {
if e.start {
if openSpans == 0 {
// Start a new span. Because for equal keys the start events
// come first, there can't be end events for this key.
// The end of the span will be adjusted as we move forward.
s = append(s, sqlbase.Span{Start: e.key, End: e.key})
}
openSpans++
} else {
openSpans--
if openSpans < 0 {
panic("end span with no spans started")
} else if openSpans == 0 {
// Adjust the end of the last span.
s[len(s)-1].End = e.key
}
}
}
if openSpans != 0 {
panic("scan ended with open spans")
}
return s
}
// selectIndex analyzes the scanNode to determine if there is an index
// available that can fulfill the query with a more restrictive scan.
//
// Analysis currently consists of a simplification of the filter expression,
// replacing expressions which are not usable by indexes by "true". The
// simplified expression is then considered for each index and a set of range
// constraints is created for the index. The candidate indexes are ranked using
// these constraints and the best index is selected. The contraints are then
// transformed into a set of spans to scan within the index.
//
// The analyzeOrdering function is used to determine how useful the ordering of
// an index is. If no particular ordering is desired, it can be nil.
//
// If preferOrderMatching is true, we prefer an index that matches the desired
// ordering completely, even if it is not a covering index.
func selectIndex(
s *scanNode, analyzeOrdering analyzeOrderingFn, preferOrderMatching bool,
) (planNode, error) {
if s.desc.IsEmpty() || (s.filter == nil && analyzeOrdering == nil && s.specifiedIndex == nil) {
// No table or no where-clause, no ordering, and no specified index.
s.initOrdering(0)
return s, nil
}
candidates := make([]*indexInfo, 0, len(s.desc.Indexes)+1)
if s.specifiedIndex != nil {
// An explicit secondary index was requested. Only add it to the candidate
// indexes list.
candidates = append(candidates, &indexInfo{
desc: &s.desc,
index: s.specifiedIndex,
})
} else {
candidates = append(candidates, &indexInfo{
desc: &s.desc,
index: &s.desc.PrimaryIndex,
})
for i := range s.desc.Indexes {
candidates = append(candidates, &indexInfo{
desc: &s.desc,
index: &s.desc.Indexes[i],
})
}
}
for _, c := range candidates {
c.init(s)
}
if s.filter != nil {
// Analyze the filter expression, simplifying it and splitting it up into
// possibly overlapping ranges.
exprs, equivalent := analyzeExpr(s.filter)
if log.V(2) {
log.Infof("analyzeExpr: %s -> %s [equivalent=%v]", s.filter, exprs, equivalent)
}
// Check to see if the filter simplified to a constant.
if len(exprs) == 1 && len(exprs[0]) == 1 {
if d, ok := exprs[0][0].(*parser.DBool); ok && bool(!*d) {
// The expression simplified to false.
return &emptyNode{}, nil
}
}
// If the simplified expression is equivalent and there is a single
// disjunction, use it for the filter instead of the original expression.
if equivalent && len(exprs) == 1 {
s.filter = joinAndExprs(exprs[0])
}
// TODO(pmattis): If "len(exprs) > 1" then we have multiple disjunctive
// expressions. For example, "a <= 1 OR a >= 5" will get translated into
// "[[a <= 1], [a >= 5]]".
//
// We currently map all disjunctions onto the same index; this works
// well if we can derive constraints for a set of columns from all
// disjunctions, e.g. `a < 5 OR a > 10`.
//
// However, we can't generate any constraints if the disjunctions refer
// to different columns, e.g. `a > 1 OR b > 1`. We would need to perform
// index selection independently for each of the disjunctive
// expressions, and we would need infrastructure to do a
// multi-index-join. There are complexities: if there are a large
// number of disjunctive expressions we should limit how many indexes we
// use.
for _, c := range candidates {
c.analyzeExprs(exprs)
}
}
if s.noIndexJoin {
// Eliminate non-covering indexes. We do this after the check above for
// constant false filter.
for i := 0; i < len(candidates); {
if !candidates[i].covering {
candidates[i] = candidates[len(candidates)-1]
candidates = candidates[:len(candidates)-1]
} else {
i++
}
}
if len(candidates) == 0 {
// The primary index is always covering. So the only way this can
// happen is if we had a specified index.
if s.specifiedIndex == nil {
panic("no covering indexes")
}
return nil, fmt.Errorf("index \"%s\" is not covering and NO_INDEX_JOIN was specified",
s.specifiedIndex.Name)
}
}
if analyzeOrdering != nil {
for _, c := range candidates {
c.analyzeOrdering(s, analyzeOrdering, preferOrderMatching)
}
}
indexInfoByCost(candidates).Sort()
if log.V(2) {
for i, c := range candidates {
log.Infof("%d: selectIndex(%s): cost=%v constraints=%s reverse=%t",
i, c.index.Name, c.cost, c.constraints, c.reverse)
}
}
// After sorting, candidates[0] contains the best index. Copy its info into
// the scanNode.
c := candidates[0]
s.index = c.index
s.isSecondaryIndex = (c.index != &s.desc.PrimaryIndex)
s.spans = makeSpans(c.constraints, c.desc.ID, c.index)
if len(s.spans) == 0 {
// There are no spans to scan.
return &emptyNode{}, nil
}
s.filter = applyConstraints(s.filter, c.constraints)
noFilter := (s.filter == nil)
s.reverse = c.reverse
var plan planNode
if c.covering {
s.initOrdering(c.exactPrefix)
plan = s
} else {
// Note: makeIndexJoin can modify s.filter.
plan = makeIndexJoin(s, c.exactPrefix)
}
// If we have no filter, we can request a single key in some cases.
if noFilter && analyzeOrdering != nil && s.isSecondaryIndex {
_, _, singleKey := analyzeOrdering(plan.Ordering())
if singleKey {
// We only need to retrieve one key, but some spans might contain no
// keys so we need to keep all of them.
for i := range s.spans {
s.spans[i].Count = 1
}
}
}
if log.V(3) {
log.Infof("%s: filter=%v", c.index.Name, s.filter)
for i, span := range s.spans {
log.Infof("%s/%d: %s", c.index.Name, i, sqlbase.PrettySpan(span, 2))
}
}
return plan, nil
}