On 2 March 2016 at 00:33, Arjen Nienhuis <a.g.nienhuis@xxxxxxxxx> wrote:
On Feb 29, 2016 22:26, "Evgeny Morozov" <evgeny.morozov+list+pgsql@xxxxxxxxxxxxxxxxxxxx> 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 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.
