Re: index-only-scan when there is an index on all columns
| От | David G. Johnston |
|---|---|
| Тема | Re: index-only-scan when there is an index on all columns |
| Дата | |
| Msg-id | CAKFQuwan3avGvqhX=jB4EcMOUeD7P2yp_8-7YpToZnx=4KXb2Q@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: index-only-scan when there is an index on all columns (Tom Lane <tgl@sss.pgh.pa.us>) |
| Ответы |
Re: index-only-scan when there is an index on all columns
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| Список | pgsql-general |
Hadi Moshayedi <hadi@citusdata.com> writes:
> I am wondering why is it not using index-only-scan (which would use the
> cache better) and instead it does a bitmap scan?
Never experiment on an empty table and assume that the resulting plan
is the same as you'd get on a large table.
In this case, not only don't you have any meaningful amount of data
loaded, but the planner can see that none of the table's pages are
marked all-visible, meaning that the "index-only" scan would degrade
to a regular indexscan, which is how it gets costed. And on a single-page
table, an indexscan is going to have a hard time beating other
alternatives.
If one runs vacuum on a table (small or otherwise) that is currently choosing an index scan as its best plan how likely is it that post-vacuum an index-only plan would be chosen if the index type and column presence conditions are met?
Also, I recall discussion that select statements will touch the visibility map (hence causing write I/O even in a read-only query) but [1] indicates that only vacuum will set them ddl will clear them.
David J.
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