Re: [Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
-+-
 Reporter:  bitrut   |Owner:  err
 Type:  Bug  |   Status:  closed
Component:  Database layer   |  Version:  1.4
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  fixed
 Keywords:  orm  | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  1
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Mariusz Felisiak ):

 In [changeset:"d2263b7b87a2af5dcd6964937b157b7e1cf2881f" d2263b7b]:
 {{{
 #!CommitTicketReference repository=""
 revision="d2263b7b87a2af5dcd6964937b157b7e1cf2881f"
 Refs #18414 -- Added tests for selected columns of sliced distinct
 querysets.
 }}}

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Re: [Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
-+-
 Reporter:  bitrut   |Owner:  err
 Type:  Bug  |   Status:  closed
Component:  Database layer   |  Version:  1.4
  (models, ORM)  |   Resolution:  fixed
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  orm  |  Needs documentation:  0
Has patch:  1|  Patch needs improvement:  1
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by Anssi Kääriäinen ):

 * status:  assigned => closed
 * resolution:   => fixed


Comment:

 In [changeset:"93cc6dcdac6fc3e506640fa38dd1798c3cd61cff"]:
 {{{
 #!CommitTicketReference repository=""
 revision="93cc6dcdac6fc3e506640fa38dd1798c3cd61cff"
 Fixed #18414 -- qs.exists() for sliced distinct queries
 }}}

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Re: [Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
-+-
 Reporter:  bitrut   |Owner:  err
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  1.4
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  orm  |  Needs documentation:  0
Has patch:  1|  Patch needs improvement:  1
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by akaariai):

 It seems this will be relatively easy to fix, just do
 {{{
 if not q.distinct:
 q.clear_select_clause()
 }}}
 instead of always doing clear_select_clause(). I'll test this & commit if
 things seem to work OK.

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Re: [Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
-+-
 Reporter:  bitrut   |Owner:  err
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  1.4
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  orm  |  Needs documentation:  0
Has patch:  1|  Patch needs improvement:  1
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by timo):

 * needs_better_patch:  0 => 1


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Re: [Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
-+-
 Reporter:  bitrut   |Owner:  err
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  1.4
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  orm  |  Needs documentation:  0
Has patch:  1|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by akaariai):

 The pull request doesn't seem to do what Luke asked - it disallows the
 usage in any filtered situation, not just in conjunction with .distinct().
 It seems likely there are users relying on this currently.

 As for fixing the distinct() + limit + .exists() - it should be done by
 not doing select distinct(1) but instead select distinct {normal select
 fields} instead. I wonder if this would be straightforward to do?

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Re: [Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
-+-
 Reporter:  bitrut   |Owner:  err
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  1.4
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  orm  |  Needs documentation:  0
Has patch:  1|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by err):

 * has_patch:  0 => 1


Comment:

 Here is my pull request: https://github.com/django/django/pull/161

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Re: [Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
-+-
 Reporter:  bitrut   |Owner:  err
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  1.4
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  orm  |  Needs documentation:  0
Has patch:  0|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by err):

 * owner:  nobody => err
 * status:  new => assigned


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Re: [Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
-+-
 Reporter:  bitrut   |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  1.4
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  orm  |  Needs documentation:  0
Has patch:  0|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by bitrut@…):

 * cc: bitrut@… (added)


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Re: [Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
-+-
 Reporter:  bitrut   |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  1.4
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  orm  |  Needs documentation:  0
Has patch:  0|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by lukeplant):

 * stage:  Unreviewed => Accepted


Comment:

 `QuerySet.exists()`  should throw an exception for this case, and possibly
 for any case involving slicing - I don't think it was designed to be
 correct in the presence of LIMIT/OFFSET. However, for the moment probably
 best to limit to the distinct()+slicing()+exists() combination.

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Re: [Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
-+-
 Reporter:  bitrut   |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  1.4
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:
 Keywords:  orm  |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by paluho@…):

 * cc: paluho@… (added)
 * needs_better_patch:   => 0
 * needs_tests:   => 0
 * needs_docs:   => 0


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[Django] #18414: queryset.exists() returns False when queryset is distinct and sliced

[Django]

#18414: queryset.exists() returns False when queryset is distinct and sliced
--+
 Reporter:  bitrut|  Owner:  nobody
 Type:  Bug   | Status:  new
Component:  Database layer (models, ORM)  |Version:  1.4
 Severity:  Normal|   Keywords:  orm
 Triage Stage:  Unreviewed|  Has patch:  0
Easy pickings:  0 |  UI/UX:  0
--+
 Consider this:
 {{{
 User.objects.count()
 3
 }}}
 This queryset
 {{{
 User.objects.all()[1:].exists()
 True
 }}}
 produces SQL
 {{{
 SELECT (1) AS "a" FROM "user" LIMIT 1 OFFSET 1
 }}}
 so the result is correct.
 But when you add `distinct()`:
 {{{
 User.objects.distinct()[1:].exists()
 False
 }}}
 the SQL is
 {{{
 SELECT DISTINCT (1) AS "a" FROM "user" LIMIT 1 OFFSET 1
 }}}
 which returns nothing, so the `exists()` returns `False` which is
 incorrect.

 `DISTINCT` narrows results to just one '''1''' and `OFFSET` omits the
 first result and goes to the next one which does not exist.
 It's because `DISTINCT` has the higher priority than `OFFSET` in SQL.

 I don't know the perfect solution. Maybe in the case of using
 `distinct()`, slicing and `exists()` together an exception should be
 thrown. Or the `exists()` function should notice this case and call
 `count()`, but this would lead to unexpected DB usage.

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