Stamatis Zampetakis created CALCITE-2624:
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Summary: Add a rule to copy a sort below a binary operator over
the left or right input
Key: CALCITE-2624
URL: https://issues.apache.org/jira/browse/CALCITE-2624
Project: Calcite
Issue Type: Improvement
Components: core
Affects Versions: 1.17.0
Reporter: Stamatis Zampetakis
Assignee: Julian Hyde
Fix For: 1.18.0
Currently, the only rule that allows a sort to traverse a binary operator is
the SortJoinTransposeRule. The rule was introduced mainly to push limits in the
case of left and right outer joins (see CALCITE-831).
I assume that the main reason that we don't have more rules is that sorts with
limits and offsets cannot be pushed safely below many binary operators.
However, in many cases, it is possible and beneficial for optimization purposes
to just push the sort (without the limit and offset) below the binary operator.
Since we do not know in advance if the binary operator preserves the sort we
cannot remove (that is why I am saying copy and not transpose) the sort
operator on top of the binary operator. The latter is not really a problem
since the SortRemoveRule can detect such cases and remove the sort if it is
redundant.
A few concrete examples where this optimization makes sense are outlined below:
* allow the sort to be later absorbed by an index scan and disappear from the
plan (Sort + Tablescan => IndexScan with RelCollation);
* allow operators that require sorted inputs to be exploited more easily
(e.g., merge join);
* allow the sort to be performed on a possibly smaller result (assuming that
the physical binary operator that is going to be used preserves the order of
left/right input and the top sort operator can be removed entirely).
I propose to add a new rule (e.g., SortCopyBelowBiRelRule,
SortBiRelCopyBelowRule, SortBiRelTransposeRule) which allows a sort to be
copied to the left or right of a BiRel operator (excluding the limit and offset
attributes) if the respective input is not already sorted. For my use case, it
would suffice to apply the rule only on joins and correlates but I don't see
why not making it a bit more general from the beginning.
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