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Consider index-only scans even when there is no matching qual or ORDER BY.
By popular demand.
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@ -317,8 +317,7 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
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bool useful_predicate;
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bool found_clause;
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bool index_is_ordered;
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bool index_only_scan = false;
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bool checked_index_only = false;
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bool index_only_scan;
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/*
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* Check that index supports the desired scan type(s)
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@ -436,17 +435,20 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
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}
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/*
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* 3. Generate an indexscan path if there are relevant restriction
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* 3. Check if an index-only scan is possible.
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*/
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index_only_scan = check_index_only(rel, index);
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/*
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* 4. Generate an indexscan path if there are relevant restriction
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* clauses in the current clauses, OR the index ordering is
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* potentially useful for later merging or final output ordering, OR
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* the index has a predicate that was proven by the current clauses.
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* the index has a predicate that was proven by the current clauses,
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* OR an index-only scan is possible.
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*/
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if (found_clause || useful_pathkeys != NIL || useful_predicate)
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if (found_clause || useful_pathkeys != NIL || useful_predicate ||
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index_only_scan)
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{
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/* First, detect whether index-only scan is possible */
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index_only_scan = check_index_only(rel, index);
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checked_index_only = true;
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ipath = create_index_path(root, index,
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restrictclauses,
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orderbyclauses,
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@ -460,7 +462,7 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
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}
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/*
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* 4. If the index is ordered, a backwards scan might be interesting.
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* 5. If the index is ordered, a backwards scan might be interesting.
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* Again, this is only interesting at top level.
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*/
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if (index_is_ordered && possibly_useful_pathkeys &&
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@ -472,9 +474,6 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
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index_pathkeys);
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if (useful_pathkeys != NIL)
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{
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if (!checked_index_only)
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index_only_scan = check_index_only(rel, index);
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ipath = create_index_path(root, index,
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restrictclauses,
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NIL,
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