static int xBestIndex(sqlite3_vtab *tab, sqlite3_index_info *pIdxInfo) {
auto *pVtab = (VirtualTable *)tab;
ConstraintSet constraints;
// Keep track of the index used for each valid constraint.
// Expect this index to correspond with argv within xFilter.
size_t expr_index = 0;
// If any constraints are unusable increment the cost of the index.
double cost = 1;
// Expressions operating on the same virtual table are loosely identified by
// the consecutive sets of terms each of the constraint sets are applied onto.
// Subsequent attempts from failed (unusable) constraints replace the set,
// while new sets of terms append.
if (pIdxInfo->nConstraint > 0) {
for (size_t i = 0; i < static_cast<size_t>(pIdxInfo->nConstraint); ++i) {
// Record the term index (this index exists across all expressions).
const auto &constraint_info = pIdxInfo->aConstraint[i];
#if defined(DEBUG)
plan("Evaluating constraints for table: " + pVtab->content->name +
" [index=" + std::to_string(i) + " column=" +
std::to_string(constraint_info.iColumn) + " term=" +
std::to_string((int)constraint_info.iTermOffset) + " usable=" +
std::to_string((int)constraint_info.usable) + "]");
#endif
if (!constraint_info.usable) {
// A higher cost less priority, prefer more usable query constraints.
cost += 10;
continue;
}
// Lookup the column name given an index into the table column set.
if (constraint_info.iColumn < 0 ||
static_cast<size_t>(constraint_info.iColumn) >=
pVtab->content->columns.size()) {
cost += 10;
continue;
}
const auto &name = pVtab->content->columns[constraint_info.iColumn].first;
// Save a pair of the name and the constraint operator.
// Use this constraint during xFilter by performing a scan and column
// name lookup through out all cursor constraint lists.
constraints.push_back(
std::make_pair(name, Constraint(constraint_info.op)));
pIdxInfo->aConstraintUsage[i].argvIndex = ++expr_index;
#if defined(DEBUG)
plan("Adding constraint for table: " + pVtab->content->name +
" [column=" + name + " arg_index=" + std::to_string(expr_index) +
"]");
#endif
}
}
pIdxInfo->idxNum = kConstraintIndexID++;
// Add the constraint set to the table's tracked constraints.
#if defined(DEBUG)
plan("Recording constraint set for table: " + pVtab->content->name +
" [cost=" + std::to_string(cost) + " size=" +
std::to_string(constraints.size()) + " idx=" +
std::to_string(pIdxInfo->idxNum) + "]");
#endif
pVtab->content->constraints[pIdxInfo->idxNum] = std::move(constraints);
pIdxInfo->estimatedCost = cost;
return SQLITE_OK;
}
