@Override
public final Expr optimize(final QueryContext qc, final VarScope scp) throws QueryException {
final Value v = initial(qc);
if (v != null && v.isEmpty() || emptyPath(v)) return optPre(qc);
// merge descendant steps
Expr e = mergeSteps(qc);
if (e == this && v != null && v.type == NodeType.DOC) {
// check index access
e = index(qc, v);
// rewrite descendant to child steps
if (e == this) e = children(qc, v);
}
// recompile path if it has changed
if (e != this) return e.compile(qc, scp);
// set atomic type for single attribute steps to speed up predicate tests
if (root == null && steps.length == 1 && steps[0] instanceof Step) {
final Step curr = (Step) steps[0];
if (curr.axis == ATTR && curr.test.kind == Kind.URI_NAME) curr.seqType(SeqType.NOD_ZO);
}
// choose best path implementation and set type information
final Path path = get(info, root, steps);
path.size = path.size(qc);
path.seqType = SeqType.get(steps[steps.length - 1].seqType().type, size);
return path;
}
/**
* If possible, returns an expression which accesses the index. Otherwise, returns the original
* expression.
*
* @param ctx query context
* @param data data reference
* @return resulting expression
* @throws QueryException query exception
*/
private Expr index(final QueryContext ctx, final Data data) throws QueryException {
// disallow relative paths and numeric predicates
if (root == null || uses(Use.POS)) return this;
// cache index access costs
IndexContext ics = null;
// cheapest predicate and step
int pmin = 0;
int smin = 0;
// check if path can be converted to an index access
for (int s = 0; s < steps.length; ++s) {
// find cheapest index access
final AxisStep stp = step(s);
if (!stp.axis.down) break;
// check if resulting index path will be duplicate free
final boolean i = pathNodes(data, s) != null;
// choose cheapest index access
for (int p = 0; p < stp.preds.length; ++p) {
final IndexContext ic = new IndexContext(ctx, data, stp, i);
if (!stp.preds[p].indexAccessible(ic)) continue;
if (ic.costs() == 0) {
if (ic.not) {
// not operator... accept all results
stp.preds[p] = Bln.TRUE;
continue;
}
// no results...
ctx.compInfo(OPTNOINDEX, this);
return Empty.SEQ;
}
if (ics == null || ics.costs() > ic.costs()) {
ics = ic;
pmin = p;
smin = s;
}
}
}
// skip if no index access is possible, or if it is too expensive
if (ics == null || ics.costs() > data.meta.size) return this;
// replace expressions for index access
final AxisStep stp = step(smin);
final Expr ie = stp.preds[pmin].indexEquivalent(ics);
if (ics.seq) {
// sequential evaluation; do not invert path
stp.preds[pmin] = ie;
} else {
// inverted path, which will be represented as predicate
AxisStep[] invSteps = {};
// collect remaining predicates
final Expr[] newPreds = new Expr[stp.preds.length - 1];
int c = 0;
for (int p = 0; p != stp.preds.length; ++p) {
if (p != pmin) newPreds[c++] = stp.preds[p];
}
// check if path before index step needs to be inverted and traversed
final Test test = DocTest.get(ctx, data);
boolean inv = true;
if (test == Test.DOC && data.meta.pathindex && data.meta.uptodate) {
int j = 0;
for (; j <= smin; ++j) {
// invert if axis is not a child or has predicates
final AxisStep s = axisStep(j);
if (s == null) break;
if (s.axis != Axis.CHILD || s.preds.length > 0 && j != smin) break;
if (s.test.test == Name.ALL || s.test.test == null) continue;
if (s.test.test != Name.NAME) break;
// support only unique paths with nodes on the correct level
final int name = data.tagindex.id(s.test.name.local());
final ObjList<PathNode> pn = data.paths.desc(name, Data.ELEM);
if (pn.size() != 1 || pn.get(0).level() != j + 1) break;
}
inv = j <= smin;
}
// invert path before index step
if (inv) {
for (int j = smin; j >= 0; --j) {
final Axis ax = step(j).axis.invert();
if (ax == null) break;
if (j != 0) {
final AxisStep prev = step(j - 1);
invSteps = Array.add(invSteps, AxisStep.get(input, ax, prev.test, prev.preds));
} else {
// add document test for collections and axes other than ancestors
if (test != Test.DOC || ax != Axis.ANC && ax != Axis.ANCORSELF)
invSteps = Array.add(invSteps, AxisStep.get(input, ax, test));
}
}
}
// create resulting expression
final AxisPath result;
final boolean simple = invSteps.length == 0 && newPreds.length == 0;
if (ie instanceof AxisPath) {
result = (AxisPath) ie;
} else if (smin + 1 < steps.length || !simple) {
result =
simple
? new AxisPath(input, ie)
: new AxisPath(input, ie, AxisStep.get(input, Axis.SELF, Test.NOD));
} else {
return ie;
}
// add remaining predicates to last step
final int ls = result.steps.length - 1;
if (ls >= 0) {
result.steps[ls] = result.step(ls).addPreds(newPreds);
// add inverted path as predicate to last step
if (invSteps.length != 0)
result.steps[ls] = result.step(ls).addPreds(Path.get(input, null, invSteps));
}
// add remaining steps
for (int s = smin + 1; s < steps.length; ++s) {
result.steps = Array.add(result.steps, steps[s]);
}
return result;
}
return this;
}
/**
* Converts descendant to child steps.
*
* @param ctx query context
* @param data data reference
* @return path
*/
Expr children(final QueryContext ctx, final Data data) {
// skip path check if no path index exists, or if it is out-of-date
if (!data.meta.uptodate || data.nspaces.globalNS() == null) return this;
Path path = this;
for (int s = 0; s < steps.length; ++s) {
// don't allow predicates in preceding location steps
final Step prev = s > 0 ? axisStep(s - 1) : null;
if (prev != null && prev.preds.length != 0) break;
// ignore axes other than descendant, or numeric predicates
final Step curr = axisStep(s);
if (curr == null || curr.axis != DESC || curr.has(Flag.FCS)) continue;
// check if child steps can be retrieved for current step
ArrayList<PathNode> pn = pathNodes(data, s);
if (pn == null) continue;
// cache child steps
final ArrayList<QNm> qnm = new ArrayList<>();
while (pn.get(0).par != null) {
QNm nm = new QNm(data.tagindex.key(pn.get(0).name));
// skip children with prefixes
if (nm.hasPrefix()) return this;
for (final PathNode p : pn) {
if (pn.get(0).name != p.name) nm = null;
}
qnm.add(nm);
pn = PathSummary.parent(pn);
}
ctx.compInfo(OPTCHILD, steps[s]);
// build new steps
int ts = qnm.size();
final Expr[] stps = new Expr[ts + steps.length - s - 1];
for (int t = 0; t < ts; ++t) {
final Expr[] preds = t == ts - 1 ? ((Preds) steps[s]).preds : new Expr[0];
final QNm nm = qnm.get(ts - t - 1);
final NameTest nt =
nm == null ? new NameTest(false) : new NameTest(nm, Mode.LN, false, null);
stps[t] = Step.get(info, CHILD, nt, preds);
}
while (++s < steps.length) stps[ts++] = steps[s];
path = get(info, root, stps);
break;
}
// check if the all children in the path exist; don't test with namespaces
if (data.nspaces.size() == 0) {
LOOP:
for (int s = 0; s < path.steps.length; ++s) {
// only verify child steps; ignore namespaces
final Step st = path.axisStep(s);
if (st == null || st.axis != CHILD) break;
if (st.test.mode == Mode.ALL || st.test.mode == null) continue;
if (st.test.mode != Mode.LN) break;
// check if one of the addressed nodes is on the correct level
final int name = data.tagindex.id(st.test.name.local());
for (final PathNode pn : data.paths.desc(name, Data.ELEM)) {
if (pn.level() == s + 1) continue LOOP;
}
ctx.compInfo(OPTPATH, path);
return Empty.SEQ;
}
}
return path;
}