0.25.2 z-order slower than 0.25.1

[aldder]

Environment

Delta-rs version: 0.25.2

Environment:

• Cloud provider: AWS

---

Bug

What happened:
I have a process that run z-order compation on a table every hour and I noticed that since the release of 0.25.2 the operation is much slower respect to the previous version, as you can see in the following monitoring chart

[ion-elgreco]

I guess our TableProvider scan is slower than ctx.read_parquet

[ion-elgreco]

@alamb sorry for pinging you but I wonder whether you might understand how these performance differences could happen?

ctx.read_parquet uses a listingTableProvider, our own DeltaTableProvider though uses a ParquetExec scan, but I don't know the exact internals of ListingTableProvider tbh

[alamb]

@alamb sorry for pinging you but I wonder whether you might understand how these performance differences could happen?

ctx.read_parquet uses a listingTableProvider, our own DeltaTableProvider though uses a ParquetExec scan, but I don't know the exact internals of ListingTableProvider tbh

Thanks for the ping @ion-elgreco -- I don't have any ideas for a report at this level

I think getting a reproducer would be the next step, and we could look at the explain plans to see if anything obvious is going on

[ion-elgreco]

@aldder can you create a reproducible example with some sample data? Easier for me to debug :)

[aldder]

@ion-elgreco mmh, unfortunately I can't seem to reproduce the error with a brand new table no matter what I try

[ion-elgreco]

@aldder how big is that existing table, how many versions, size of log files, size of checkpoint, amount of rows, columns?

[aldder]

@ion-elgreco

the table has ~25k versions right now
checkpoints size is ~25-30MB

schema is:
x1: string
x2: timestamp[us, tz=UTC]
x3: timestamp[us, tz=UTC]
x4: double

I don't know how many rows it has tbh, but i would say hundred millions

[ion-elgreco]

@ion-elgreco

the table has ~25k versions right now
checkpoints size is ~25-30MB

schema is:
x1: string
x2: timestamp[us, tz=UTC]
x3: timestamp[us, tz=UTC]
x4: double

I don't know how many rows it has tbh, but i would say hundred millions

Ok, Ill make a test branch, I have a theory that might help. Could you compile that wheel then and test it with your table?

[aldder]

I have never used Rust before, so I don't know the procedure to compile the library. If you can give me steps to follow I would really appreciate it

[aldder]

Ok, finally I was able to reproduce the issue. Apparently it's something related to the number of partitions since I just needed to increase these number to obtain the desired effect.

import polars as pl
import deltalake
from deltalake import DeltaTable, Schema, Field
from time import perf_counter
from tqdm.auto import tqdm

print(deltalake.__version__)

DeltaTable.create('tmp', schema=Schema([
        Field('foo', 'string'),
        Field('num', 'double'),
        Field('time', 'timestamp')
    ]),
    partition_by=['foo']
)

for i in tqdm(range(50)):
    df = pl.DataFrame({
        'foo': pl.int_range(0, 500, eager=True).cast(pl.String),
        'num': pl.int_range(i, i + 500, eager=True),
        'time': pl.datetime_range(pl.datetime(2021, 1, 1), pl.datetime(2021, 1, 1) + pl.duration(hours=499), '1h', time_zone='UTC', eager=True)
    })

    df.write_delta(
        target='tmp',
        mode='append'
    )

dt = DeltaTable('tmp')
tic = perf_counter()
dt.optimize.z_order(['time'])
toc = perf_counter()

print(f"Optimization time: {toc-tic}")

With version 0.25.2

0.25.2
100%|█████████████████████████████████████████████████████████| 50/50 [00:53<00:00,  1.07s/it]
Optimization time: 29.001007099999697

with version 0.25.1

0.25.1
100%|█████████████████████████████████████████████████████████| 50/50 [00:52<00:00,  1.05s/it]
Optimization time: 19.724042600006214

[ion-elgreco]

Ok, finally I was able to reproduce the issue. Apparently it's something related to the number of partitions since I just needed to increase these number to obtain the desired effect.

import polars as pl
import deltalake
from deltalake import DeltaTable, Schema, Field
from time import perf_counter
from tqdm.auto import tqdm

print(deltalake.__version__)

DeltaTable.create('tmp', schema=Schema([
        Field('foo', 'string'),
        Field('num', 'double'),
        Field('time', 'timestamp')
    ]),
    partition_by=['foo']
)

for i in tqdm(range(50)):
    df = pl.DataFrame({
        'foo': pl.int_range(0, 500, eager=True).cast(pl.String),
        'num': pl.int_range(i, i + 500, eager=True),
        'time': pl.datetime_range(pl.datetime(2021, 1, 1), pl.datetime(2021, 1, 1) + pl.duration(hours=499), '1h', time_zone='UTC', eager=True)
    })

    df.write_delta(
        target='tmp',
        mode='append'
    )

dt = DeltaTable('tmp')
tic = perf_counter()
dt.optimize.z_order(['time'])
toc = perf_counter()

print(f"Optimization time: {toc-tic}")

With version 0.25.2

0.25.2
100%|█████████████████████████████████████████████████████████| 50/50 [00:53<00:00,  1.07s/it]
Optimization time: 29.001007099999697

with version 0.25.1

0.25.1
100%|█████████████████████████████████████████████████████████| 50/50 [00:52<00:00,  1.05s/it]
Optimization time: 19.724042600006214

Thanks this is very useful to narrow the issue down! I'll try to dive deeper into it which part in the DeltaScan makes this more slower

[ion-elgreco]

Actually I can't reproduce this.. this is main with release mode:

Optimization time: 1.1585120419913437

And 0.25.1:
Optimization time: 1.0465130830125418

[ion-elgreco]

@aldder I have a small fix that slightly brings it closer to 0.25.1 for me. You can try it out if you want: https://github.com/ion-elgreco/delta-rs/tree/fix/optimize_scan

Make sure to build the wheel in --release mode

[aldder]

@ion-elgreco I tested the changes, but they don't seem to have changed much in substance

0.25.1
Optimization time: 45.88432119999197

0.25.2 (fix)
Optimization time: 59.264429899980314

I did several tests to confirm 30% more slowness than the previous version