Samyak2 opened a new pull request, #9680:
URL: https://github.com/apache/arrow-rs/pull/9680
# Which issue does this PR close?
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- Closes None
# Rationale for this change
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We internally found `RowNumberReader` to be a hot path in some of our
queries. Flamegraphs showed ~70% of the cpu time taken by methods in
`RowNumberReader`.
These can be made an order of magnitude faster (benchmarks below).
# What changes are included in this PR?
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- Instead of storing an iterator over individual row numbers, we now store a
vec of ranges.
- These ranges are not materialized into a fully array until needed.
- `read_records` was previously linear in terms of number of rows read.
- Now it's close to constant since one batch (8192 rows) usually is
satisfied by one row range (which comes from a row group).
- Same for `skip_records`
- `consume_batch` is still linear in terms of rows, but it is faster since
it can pre-allocate the output vec.
- Previously, the `Flatten` iter would have prevented it pre-allocating
(it's not an `ExactSizeIterator`).
# Are these changes tested?
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- Yes, added more unit tests
- I have some benchmarks at https://github.com/Samyak2/arrow-rs/pull/1, but
they need `RowNumberReader` to be pub, so I've not included them here
Before:
```
Benchmarking row_number_read_consume: Warming up for 3.0000 s
Warning: Unable to complete 100 samples in 5.0s. You may wish to increase
target time to 7.0s, enable flat sampling, or reduce sample count to 50.
row_number_read_consume time: [1.3915 ms 1.3967 ms 1.4035 ms]
Found 11 outliers among 100 measurements (11.00%)
1 (1.00%) low severe
1 (1.00%) low mild
5 (5.00%) high mild
4 (4.00%) high severe
row_number_skip_and_read
time: [716.61 µs 718.14 µs 719.91 µs]
Found 6 outliers among 100 measurements (6.00%)
1 (1.00%) low severe
1 (1.00%) low mild
3 (3.00%) high mild
1 (1.00%) high severe
```
After:
```
row_number_read_consume time: [159.00 µs 160.81 µs 162.68 µs]
change: [−88.900% −88.721% −88.505%] (p = 0.00 <
0.05)
Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
1 (1.00%) low mild
2 (2.00%) high mild
row_number_skip_and_read
time: [79.057 µs 79.924 µs 80.846 µs]
change: [−89.025% −88.865% −88.712%] (p = 0.00 <
0.05)
Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
2 (2.00%) high mild
```
Ranging from **8.6x to 8.9x faster**!
# Are there any user-facing changes?
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No
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