/**
* Get the index that matches the ORDER BY list, if one exists. This is to avoid running a
* separate ORDER BY if an index can be used. This is specially important for large result sets,
* if only the first few rows are important (LIMIT is used)
*
* @return the index if one is found
*/
private Index getSortIndex() {
if (sort == null) {
return null;
}
ArrayList<Column> sortColumns = New.arrayList();
for (int idx : sort.getIndexes()) {
if (idx < 0 || idx >= expressions.size()) {
throw DbException.getInvalidValueException("ORDER BY", idx + 1);
}
Expression expr = expressions.get(idx);
expr = expr.getNonAliasExpression();
if (expr.isConstant()) {
continue;
}
if (!(expr instanceof ExpressionColumn)) {
return null;
}
ExpressionColumn exprCol = (ExpressionColumn) expr;
if (exprCol.getTableFilter() != topTableFilter) {
return null;
}
sortColumns.add(exprCol.getColumn());
}
Column[] sortCols = sortColumns.toArray(new Column[sortColumns.size()]);
int[] sortTypes = sort.getSortTypes();
if (sortCols.length == 0) {
// sort just on constants - can use scan index
return topTableFilter.getTable().getScanIndex(session);
}
ArrayList<Index> list = topTableFilter.getTable().getIndexes();
if (list != null) {
for (int i = 0, size = list.size(); i < size; i++) {
Index index = list.get(i);
if (index.getCreateSQL() == null) {
// can't use the scan index
continue;
}
if (index.getIndexType().isHash()) {
continue;
}
IndexColumn[] indexCols = index.getIndexColumns();
if (indexCols.length < sortCols.length) {
continue;
}
boolean ok = true;
for (int j = 0; j < sortCols.length; j++) {
// the index and the sort order must start
// with the exact same columns
IndexColumn idxCol = indexCols[j];
Column sortCol = sortCols[j];
if (idxCol.column != sortCol) {
ok = false;
break;
}
if (idxCol.sortType != sortTypes[j]) {
// NULL FIRST for ascending and NULLS LAST
// for descending would actually match the default
ok = false;
break;
}
}
if (ok) {
return index;
}
}
}
if (sortCols.length == 1 && sortCols[0].getColumnId() == -1) {
// special case: order by _ROWID_
Index index = topTableFilter.getTable().getScanIndex(session);
if (index.isRowIdIndex()) {
return index;
}
}
return null;
}
