From: | Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us> |
---|---|
To: | vadim(at)sable(dot)krasnoyarsk(dot)su (Vadim B(dot) Mikheev) |
Cc: | daveh(at)insightdist(dot)com, hackers(at)postgreSQL(dot)org |
Subject: | Re: [HACKERS] Re: [BUGS] General Bug Report: palloc fails with lots of ANDs and ORs |
Date: | 1998-03-16 05:09:01 |
Message-ID: | 199803160509.AAA22496@candle.pha.pa.us |
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Thread: | |
Lists: | pgsql-hackers |
Added to TODO list.
>
> > >
> > > select count(*) from outlet
> > > where
> > > (number = 1 and number = 1 and number = 1) or
> > > (number = 1 and number = 1 and number = 1) or
> > > (number = 1 and number = 1 and number = 1) or
> > > (number = 1 and number = 1 and number = 1) or
> > > (number = 1 and number = 1 and number = 1) or
> > > (number = 1 and number = 1 and number = 1) or
> > > (number = 1 and number = 1 and number = 1) or
> > > (number = 1 and number = 1 and number = 1) or
> > > (number = 1 and number = 1 and number = 1);
> > >
> > > Not pretty but it makes the point. Take out two OR clauses and the query
> > > works fine (but a bit slow).
> > >
> > > The above query is all it takes to use up all 130000 Kbytes of memory.
> > > And, since the query takes a long time to finally fail, I was able to
> > > observe the memory consumption.
>
> Optimizator tries to transform qual above into AND clause with
> 3 (# of and-ed clauses) ^ 9 (# of OR-s) = 19683 args (each arg
> is OR clause with 9 op. expressions. My estimation for current
> cnfify() code is that this will require =~ 500Mb of memory :)
> I made little changes - just to free memory when it's possible:
>
> current code with free-ing
>
> 6 ORs 14.3 Mb 4.3 Mb
> 7 ORs 53 Mb 10.3 Mb
> 8 ORs estimation: ~ 160 Mb 30.6 MB
>
> I'm not sure should I aplly my changes or not - it doesn't fix
> problem, just reduces memory impact. It obviously can't help you,
> David, in your real example (3 ^ 28 = 22876792454961 clauses - he he :).
>
> Resume: cnfify() makes mathematically strong but in some cases
> practically unwise work. I can't fix this for 6.3
>
> Vadim
>
--
Bruce Momjian | 830 Blythe Avenue
maillist(at)candle(dot)pha(dot)pa(dot)us | Drexel Hill, Pennsylvania 19026
+ If your life is a hard drive, | (610) 353-9879(w)
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