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 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 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. ------- 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; -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
