Github user jianqiao commented on the issue:
https://github.com/apache/incubator-quickstep/pull/300
@dylanpbacon Hi Dylan, I updated the LIP related stuff and put it in this
[branch](https://github.com/apache/incubator-quickstep/tree/Hash-Join-Fuse),
you may just fetch the changes to reset your repo's `Hash-Join-Fuse` branch,
then `git push -f`, then we can merge this PR.
**Note:**
The optimization is enabled by default, and I added an extra gflag
`fuse-hash-select-threshold` that is set to one million (`1000000u`) by
default. A fusion transformation is applied only when the estimated cardinality
of the build-side selection is _greater_ than the threshold.
Overall speaking, the fuse-hash-select optimization is especially
beneficial when _the build-side selection has large output cardinality_ (e.g.
TPC-H Q21), and the benefits come from two aspects:
(1) smaller memory footprint,
(2) avoiding materialization of the selection's output.
However, due to some issues in current implementation of HashJoinOperator
(and buffer manager), the fusion may slow down some queries (e.g. TPC-H Q02,
400ms -> 700ms with LIP fixed). The `fuse-hash-select-threshold` prevents those
situations.
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