Horton

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

Todd A. Cook <tcook@blackducksoftware.com>

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From: "Todd A. Cook" <tcook@blackducksoftware.com>
To: Andres Freund <andres@anarazel.de>, Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Thomas Munro <thomas.munro@enterprisedb.com>, PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Date: 2017-12-06T21:38:18Z
Lists: pgsql-bugs 
On 12/06/17 16:12, Andres Freund wrote:
> 
>> The datasets I shared are somewhat extreme in the sense that there are
>> many contiguous sequences of hash values, but it only takes one such
>> sequence with at least SH_GROW_MAX_MOVE values to trigger the issue. So
>> the hash table may still be perfectly fine for most keys, and only
>> slightly slower for the keys in the sequence.
> 
> Meh, we're talking about adversarial attacks here.

Hmmmmm...

I found this problem when I dropped 10.1 into a test environment to see
what would happen.  There was no deliberate attempt to break anything.

-- todd

Commits

  1. Improve bit perturbation in TupleHashTableHash.

  2. Prevent growth of simplehash tables when they're "too empty".

  3. Add stack-overflow guards in set-operation planning.