#17198: In admin results can be omitted due to pagination and inadequate
ordering
clauses
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Reporter: lukeplant | Owner: nobody
Type: Bug | Status: new
Component: contrib.admin | Version:
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by akaariai):
I tested this on PostgreSQL 9.0. The only case where appending pk to the
order by clause could have serious performance implications is if you
have:
- a lot of rows
- an index on the sort_col
- no index on sort_col, pk
The test shows that in the above case you will get a limit 20 index scan
if you order by the sort_col, but you will get a sequential scan + sort if
you order by sort_col, pk. Now, if you have a lot of rows, the sort is
going to be expensive. Of course, dropping the index on sort_col and
creating a sort_col, pk index will solve that performance issue. If you
don't have any index on the sort_col to begin with, you will get a seq
scan + sort anyways.
For a 100000 rows table I measured a difference of 1ms <-> 75ms. If you
have million rows, then the difference was 1ms <-> 1 seconds. The
difference is likely bigger if you have lots of columns (only 2 in the
test). In the worst case you will need to do disk IO, and the difference
will explode to much bigger numbers.
The fix is an index on sort_col, pk in the rare cases where that is
needed.
--
Ticket URL: <https://code.djangoproject.com/ticket/17198#comment:2>
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