Remove ORDER BY pk from subqueryload, selectinload

Modified the queries used by subqueryload and selectinload to no longer
ORDER BY the primary key of the parent entity;  this ordering was there to
allow the rows as they come in to be copied into lists directly with a
minimal level of Python-side collation.   However, these ORDER BY clauses
can negatively impact the performance of the query as in many scenarios
these columns are derived from a subquery or are otherwise not actual
primary key columns such that SQL planners cannot make use of indexes. The
Python-side collation uses the native itertools.group_by() to collate the
incoming rows, and has been modified to allow multiple
row-groups-per-parent to be assembled together using list.extend(), which
should still allow for relatively fast Python-side performance.  There will
still be an ORDER BY present for a relationship that includes an explicit
order_by parameter, however this is the only ORDER BY that will be added to
the query for both kinds of loading.

Fixes: #5162
Change-Id: I8befd1303c1af7cc24cbf005f39bc01c8b2745f3

0 files changed, 0 insertions, 0 deletions