Re: Optimize partial TOAST decompression

On Sat, Jul 06, 2019 at 02:27:56AM +0800, Binguo Bao wrote:
>Hi, Tomas!
>Thanks for your testing and the suggestion.
>
>That's quite bizarre behavior - it does work with a prefix, but not with
>> suffix. And the exact ERROR changes after the prefix query.
>
>
>I think bug is caused by "#2 0x00000000004c3b08 in
>heap_tuple_untoast_attr_slice (attr=<optimized out>, sliceoffset=0,
>slicelength=-1) at tuptoaster.c:315",
>since I ignore the case where slicelength is negative, and I've appended
>some comments for heap_tuple_untoast_attr_slice for the case.
>
>FWIW the new code added to this
>> function does not adhere to our code style, and would deserve some
>> additional explanation of what it's doing/why. Same for the
>> heap_tuple_untoast_attr_slice, BTW.
>
>
>I've added more comments to explain the code's behavior.
>Besides, I also modified the macro "TOAST_COMPRESS_RAWDATA" to
>"TOAST_COMPRESS_DATA" since
>it is used to get toast compressed data rather than raw data.
>

Thanks, this seems to address the issue - I can no longer reproduce the
crashes, allowing the benchmark to complete. I'm attaching the script I
used and spreadsheet with a summary of results.

For the cases I've tested (100k - 10M values, different compressibility,
different prefix/length values), the results are kinda mixed - many
cases got much faster (~2x), but other cases got slower too. We're
however talking about queries taking a couple of miliseconds, so in
absolute numbers the differences are small.

That does not mean the optimization is useless - but the example shared
at the beginning of this thread is quite extreme, as the values are
extremely compressible. Each value is ~38MB (in plaintext), but a table
with 100 such values has only ~40MB. Tha's 100:1 compression ratio,
which I think is not typical for real-world data sets.

The data I've used are less extreme, depending on the fraction of random
data in values.

I went through the code too. I've reworder a couple of comments and code
style issues, but there are a couple of more serious issues.

1) Why rename TOAST_COMPRESS_RAWDATA to TOAST_COMPRESS_DATA?

This seems unnecessary, and it discards the clear hint that it's about
accessing the *raw* data, and the relation to TOAST_COMPRESS_RAWSIZE.
IMHO we should keep the original naming.

2) pglz_maximum_compressed_size signatures are confusing

There are two places with pglz_maximum_compressed_size signature, and
those places are kinda out of sync when it comes to parameter names:

int32
pglz_maximum_compressed_size(int32 raw_slice_size,
int32 total_compressed_size)

extern
int32 pglz_maximum_compressed_size(int32 raw_slice_size,
int32 raw_size);

Also, pg_lzcompress.c has no concept of a "slice" because it only deals
with simple compression, slicing is responsibility of the tuptoaster. So
we should not mix those two, not even in comments.

I propose tweaks per the attached patch - I think it makes the code
clearer, and it's mostly cosmetic stuff. But I haven't tested the
changes beyond "it compiles".

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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