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Re: Analyzing foreign tables & memory problems

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Analyzing foreign tables & memory problems
Date: 2012-04-30 15:29:17
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Lists: pgsql-hackers
On Mon, Apr 30, 2012 at 3:24 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> "Albe Laurenz" <laurenz(dot)albe(at)wien(dot)gv(dot)at> writes:
>> During ANALYZE, in analyze.c, functions compute_minimal_stats
>> and compute_scalar_stats, values whose length exceed
>> WIDTH_THRESHOLD (= 1024) are not used for calculating statistics
>> other than that they are counted as "too wide rows" and assumed
>> to be all different.
>> This works fine with regular tables; values exceeding that threshold
>> don't get detoasted and won't consume excessive memory.
>> With foreign tables the situation is different.  Even though
>> values exceeding WIDTH_THRESHOLD won't get used, the complete
>> rows will be fetched from the foreign table.  This can easily
>> exhaust maintenance_work_mem.
> I'm fairly skeptical that this is a real problem

AFAIK its not possible to select all columns from an Oracle database.
If you use an unqualified LONG column as part of the query then you
get an error.

So there are issues with simply requesting data for analysis.

 Simon Riggs         
 PostgreSQL Development, 24x7 Support, Training & Services

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