From: Tom Lane <tgl@sss.pgh.pa.us>
To: Todd A. Cook <tcook@blackducksoftware.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop
Date: Wed, 06 Dec 2017 12:14:24 -0500
Message-ID: <25629.1512580464@sss.pgh.pa.us> (raw)
In-Reply-To: <1773c302-2fb1-749f-8ccc-62b23617c5ac@blackducksoftware.com>
References: <20171127185700.1470.20362@wrigleys.postgresql.org>
<fd924fd9-8f27-d945-04a9-57b503896107@2ndquadrant.com>
<7a22f496-92b5-d199-984e-183965918304@blackducksoftware.com>
<20171127221041.mezzysc4zcfsqkyc@alap3.anarazel.de>
<CAEepm=3SC9zYLKhvtZbsnwf9vEpi96WCGz1rdGLofuzBau=fqA@mail.gmail.com>
<17357.1511841783@sss.pgh.pa.us>
<9cd29b81-7ba1-91a2-f491-9014f4ac87e6@blackducksoftware.com>
<2a76784c-06cb-46ec-35be-9895d144b839@2ndquadrant.com>
<219b37b2-d740-d04a-203a-49ae422028f7@blackducksoftware.com>
<c15378b3-d4aa-a5c0-5538-dd52904dcbc6@2ndquadrant.com>
<1773c302-2fb1-749f-8ccc-62b23617c5ac@blackducksoftware.com>
"Todd A. Cook" <tcook@blackducksoftware.com> writes:
> If hashint8() is ultimately invoked by TupleHashTableHash() in execGroups.c,
> it might be magnifying the difficulties here. The least significant bits,
> which are used as the bucket number in simplehash.h, are not very well
> distributed:
> select val, to_hex(val), to_hex(hashint8(val)) from temp_f_03 limit 15 ;
> val | to_hex | to_hex
> ----------------------+------------------+----------
> 4444319256653758784 | 3dad64d121468140 | 805ffffe
> 554179993563924608 | 7b0d7c49a018880 | 84dffffb
> -3383965646518123872 | d109bd2c6b2982a0 | 9c3ffff7
> -4706811054739454944 | beae0c48915f8420 | 191ffff6
> 618200668902031424 | 8944a3ba5d08040 | 2a3ffff0
> 5074043922812601024 | 466aa01079f982c0 | 7effffee
> -8783188184262212928 | 861bd8e1b9a482c0 | a6bfffea
> -4597800992953433792 | c031545b6b128140 | b1dfffea
> 8563040839807173408 | 76d608465dde8320 | 7d9fffe6
> 6092569112843158816 | 548d27540c888520 | 6f9fffe6
> -7313351060369079936 | 9a81c1f558f98180 | 68ffffe5
> -1786712428165627488 | e7345283536981a0 | 73ffffe5
> -6153596242570280896 | aa9a08d20e6b8040 | ac3fffd8
> 88426174078092128 | 13a272306c58360 | b57fffd8
> -5305589938458295680 | b65ec20faa4e8280 | ba9fffd3
Checking out hashint8() on random data shows no such obvious fault.
You've apparently got a data set that exposes a weakness in hashint8,
but it's not very clear what that is.
In any case, the hashtable code needs to not fall over in the
presence of a lot of collisions, regardless of the exact reason
for there being a lot.
regards, tom lane
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To: pgsql-bugs@postgresql.org
Cc: tgl@sss.pgh.pa.us, tcook@blackducksoftware.com, tomas.vondra@2ndquadrant.com, thomas.munro@enterprisedb.com, andres@anarazel.de
Subject: Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an infinite loop
In-Reply-To: <25629.1512580464@sss.pgh.pa.us>
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