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Thread: substring on bit(n) and bytea types is slow

substring on bit(n) and bytea types is slow

From
Evgeny Morozov
Date:
Hi,

Queries like this:

SELECT substring(bitarray from (32 * (n - 1) + 1) for 32) -- bitarray is a column of type bit(64000000)
FROM array_test_bit
JOIN generate_series(1, 10000) n ON true;

SELECT substring(bytearr from (8 * (n - 1) + 1) for 8) -- bytearr is a column of type bytea
FROM array_test_bytea
JOIN generate_series(1, 10000) n ON true;

...are really slow. These take over a minute each and a postgres backend process uses 100% of a CPU while the query runs. The same thing in SQL Server 2014 (using varbinary(max) columns) runs fast - about 20 seconds for 4 million rows. Are byte/bit arrays just inherently slow in Postgres? Or is substring the wrong function to use for them?

The context is that I want to efficiently store many integers. The obvious answer is integer[], but most of my integers can fit into less than 32 bits, so I'd like to see if I can pack them more efficiently.

Regards,

Evgeny Morozov

Re: substring on bit(n) and bytea types is slow

From
Arjen Nienhuis
Date:


On Feb 29, 2016 22:26, "Evgeny Morozov" <evgeny.morozov+list+pgsql@shift-technology.com> wrote
> SELECT substring(bitarray from (32 * (n - 1) + 1) for 32) -- bitarray is a column of type bit(64000000)
> FROM array_test_bit
> JOIN generate_series(1, 10000) n ON true;

Substring on a bit string is not optimized for long TOASTed values. Substring on text is optimized for that. The current code fetches the whole 8MB from the table every time.

Re: substring on bit(n) and bytea types is slow

From
Evgeny Morozov
Date:
On 2 March 2016 at 00:33, Arjen Nienhuis <a.g.nienhuis@gmail.com> wrote:


On Feb 29, 2016 22:26, "Evgeny Morozov" <evgeny.morozov+list+pgsql@shift-technology.com> wrote
> SELECT substring(bitarray from (32 * (n - 1) + 1) for 32) -- bitarray is a column of type bit(64000000)
> FROM array_test_bit
> JOIN generate_series(1, 10000) n ON true;

Substring on a bit string is not optimized for long TOASTed values. Substring on text is optimized for that. The current code fetches the whole 8MB from the table every time.

I see, thanks. Is there a better way to pack a large number of integers efficiently with reasonable read/write performance?

I tried arrays bit varying, which seemed perfect, but in practice when I stored 4M integers in it, each one taking as few bits as possible, the table takes 13MB - same as if I just store all of them as bit(24). In fact, an array of 4M bit(10) integers also takes 13MB. bit(8) takes only 0.7 MB. bit(9) is where things get weird: for integer 1 to 4M it takes 13MB, but if I multiple them by 2 (i.e. store 4M even integers) it takes 0.7MB! So there must be some kind of compression going on there, but I don't understand how it works.

Re: substring on bit(n) and bytea types is slow

From
Evgeny Morozov
Date:
On 2 March 2016 at 00:33, Arjen Nienhuis <a.g.nienhuis@gmail.com> wrote:


On Feb 29, 2016 22:26, "Evgeny Morozov" <evgeny.morozov+list+pgsql@shift-technology.com> wrote
> SELECT substring(bitarray from (32 * (n - 1) + 1) for 32) -- bitarray is a column of type bit(64000000)
> FROM array_test_bit
> JOIN generate_series(1, 10000) n ON true;

Substring on a bit string is not optimized for long TOASTed values. Substring on text is optimized for that. The current code fetches the whole 8MB from the table every time.

I see, thanks. Is there a better way to pack a large number of integers efficiently with reasonable read/write performance?

I tried arrays bit varying, which seemed perfect, but in practice when I stored 4M integers in it, each one taking as few bits as possible, the table takes 13MB - same as if I just store all of them as bit(24). In fact, an array of 4M bit(10) integers also takes 13MB. bit(8) takes only 0.7 MB. bit(9) is where things get weird: for integer 1 to 4M it takes 13MB, but if I multiple them by 2 (i.e. store 4M even integers) it takes 0.7MB! So there must be some kind of compression going on there, but I don't understand how it works.