Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop

Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> writes:
> When SH_INSERT tries to insert that final extra value, insertdist
> keeps exceeding SH_GROW_MAX_DIB (25) no matter how many times we
> double the size (at least until my computer gives up, somewhere around
> 11 doublings and 75GB of virtual memory). If you set SH_GROW_MAX_DIB
> to 26 then it succeeds, but I guess some other attack could be crafted
> for that. What is the theory behind this parameter?

You beat me to it --- after looking at simplehash.h I'd guessed that
either the SH_GROW_MAX_DIB or SH_GROW_MAX_MOVE code path was causing
an infinite loop, but I'd not gotten to determining which one yet.

I'd ask what's the theory behind SH_GROW_MAX_MOVE, as well. Neither
of them are obviously loop-proof.

Note that the sample data has a lot of collisions:

regression=# select hashint8(val), count(*) from reproducer group by 1 order by 2 desc;
hashint8 | count
-------------+-------
441526644 | 2337
-1117776826 | 1221
-1202007016 | 935
-2068831050 | 620
1156644653 | 538
553783815 | 510
259780770 | 444
371047036 | 394
915722575 | 359
... etc etc ...

It's evidently more complicated than just that the code fails with
more than SH_GROW_MAX_DIB duplicate hashcodes, but I suspect not
by a lot. There needs to be a safety valve that prevents letting
the hash fill factor approach zero, which is what's happening in
this test case.

(I wonder whether these loops oughtn't contain CHECK_FOR_INTERRUPTS,
btw.)

regards, tom lane