Beispiel #1
0
// 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
}
Beispiel #2
0
// 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
}