On Jun 22, 2016, at 4:25 AM, Erik Gustafson wrote:
> don't you want an index on t_a2b.col_a, maybe partial where col_a=1 ?
that table has indexes on all columns. they're never referenced because the rows are so short. this was just an example query too, col_a has 200k variations
After a lot of testing, I think I found a not-bug but possible area-for-improvement in the planner when joining against a table for filtering (using my production 9.5.2 box)
I checked a query
What query? A self-contained email would be nice.
against multiple possible indexes using the related columns. only one of indexes was on the table for each series of tests, and I analyzed the table after the drop/create of indexes.
Note 1: The only time an index-only scan is used, is on this form:
CREATE INDEX idx_partial_fkey_id_partial ON table_a(fkey_1, id, col_partial) WHERE col_partial IS NOT FALSE;
Omitting the col_partial from being indexed will trigger a Bitmap Heap Scan on the full table with a recheck condition:
CREATE INDEX idx_partial_fkey_id ON table_a(fkey_1, id) WHERE col_partial IS NOT FALSE;
This shouldn't be necessary. the planner knew that `col_partial` fulfilled the WHERE clause when it used the index, but scanned the table to check it anyways.
On most tables the heap scan was negligible, but on a few larger tables it accounted a 20% increase in execution.
Note especially:
"Visibility information is not stored in index entries, only in heap entries; ..."
The check against the heap isn't for the truthiness of the predicate but the visibility of the row.
Note 2:
This is odd, but this index is used by the planner:
CREATE INDEX idx_partial_fkey_id ON table_a(fkey_1, id) WHERE col_partial IS NOT FALSE;
but this index is never used:
CREATE INDEX idx_partial_id_fkey ON table_a(id, fkey_1) WHERE col_partial IS NOT FALSE;
I honestly don't know why the second index would not be used. The query time doubled without it when run on a table with 6million rows and about 20 columns.
This one requires knowledge of the query; but I am not surprised that reversing the order of columns in a b-tree index has an impact.
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The indexes I tested on:
CREATE INDEX idx_fkey_1 ON table_a(fkey_1);
CREATE INDEX idx_partial_fkey ON table_a(fkey_1) WHERE col_partial IS NOT FALSE;
CREATE INDEX idx_partial_fkey_id ON table_a(fkey_1, id) WHERE col_partial IS NOT FALSE;
CREATE INDEX idx_partial_id_fkey ON table_a(id, fkey_1) WHERE col_partial IS NOT FALSE;
CREATE INDEX idx_partial_fkey_partial ON table_a(fkey_1, col_partial) WHERE col_partial IS NOT FALSE;
CREATE INDEX idx_partial_fkey_id_partial ON table_a(fkey_1, id, col_partial) WHERE col_partial IS NOT FALSE;
All at once?
David J.
