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Optimize Cursor.fetchall #62

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merged 3 commits into from
Oct 13, 2023
Merged

Optimize Cursor.fetchall #62

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a417132
Optimize Cursor.fetchall
rostan-t Jan 10, 2023
d6a0afb
Set the results to `[]` instead of `None` after fetchall
rostan-t May 24, 2023
3f358ca
Merge branch 'master' into cursor-fetchall-perf
xiangfu0 Oct 13, 2023
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10 changes: 6 additions & 4 deletions pinotdb/db.py
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Original file line number Diff line number Diff line change
Expand Up @@ -506,11 +506,13 @@ def fetchmany(self, size=None):
def fetchall(self):
"""
Fetch all (remaining) rows of a query result, returning them as a
sequence of sequences (e.g. a list of tuples). Note that the cursor's
arraysize attribute can affect the performance of this operation.
sequence of sequences (e.g. a list of tuples).
"""
return list(self)


results = self._results
self._results = None
return results
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IIUC, this change may not be safe

  1. this change the value of self._results to None, will affect other methods which need this result
  2. the returned result is changed, previously, we change self to a list, now it's just part of self (i.e., self._results).

Can we try to fix those?

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To keep backward compatibility, maybe we can add a new method fetchresults? and keep the existing one unchanged?

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@rostan-t rostan-t May 24, 2023 •
edited
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Thanks for taking the time to review this pull request.

It is actually an expected behaviour that a call to fetchall affects subsequent calls to other methods. In fact, this is already the case since returning list(self) will call __iter__, which will itself call fetchone until the list is emptied.

To sum up, after a call to fetchall, fetchone has been called n times with n the size of the resulting list and self._results is empty.

There is actually a mistake in my code, I should have set self._results to [] instead of None1. Appart from this, the bahavior is strictly the same, except from the fact that the result is directly returned as is instead of resulting from multiple calls to fetchone.

Footnotes

  1. Edit: this is now fixed ↩

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@RostanTabet it might be useful to add a simple unit test to show that this is not an incompatible change in the function's behavior. Can you please add a unit test?

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The method is already tested in :

pinot-dbapi/tests/unit/test_db.py

Line 618 in ef93a90

def test_fetches_all_results(self):

It needs more ?


@check_result
@check_closed
def fetchwithschema(self):
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