From: Tom Lane <tgl@sss.pgh.pa.us>
To: Rod Taylor <rbt@rbt.ca>
Cc: A.M. <agentm@cmu.edu>
Cc: pgsql-sql@postgresql.org
Subject: Re: Column limits in table/ views
Date: Mon, 09 Jun 2003 10:22:23 -0400
Message-ID: <17237.1055168543@sss.pgh.pa.us> (raw)
In-Reply-To: <1054993688.23997.26.camel@jester>
References: <945001F0-986A-11D7-AAB2-0030657192DA@cmu.edu>
<1054993688.23997.26.camel@jester>
Rod Taylor <rbt@rbt.ca> writes:
> On Fri, 2003-06-06 at 18:02, A.M. wrote:
>> I have 560 columns of NUMERIC(10,14). To not run up against max column=20
>> restraints, I split the information into two tables. Does the column=20
>> limit on tables imply the same limit for views or selects or could I=20
>> potentially select a row across both tables and make a view that hides=20
>> the split?
> The limit is documented as having to do with the size of a block. The
> largest a single row can become is 8k with primitive types (text, and
> other variable length types will 'overflow' into another area -- see
> TOAST).
> Selects should be able to return more columns than that. Both views and
> tables create a ROW TYPE to represent it, which is where the limitation
> is. Thus, I would not expect you to be able to create a view.
But a view row is never stored on disk, so the block-size limit doesn't
come into play. You will still be constrained by the max column count
(1600), because that comes from the tuple header layout --- the size of
the header plus null bitmap has to fit into a uint8 field. But a view
with 560 columns would work.
I think some performance issues might come up if you actually try to
select all the columns at once, because there are places whose behavior
is O(N^2) in the number of columns. Dunno whether this effect will be
noticeable with ~560 columns though.
regards, tom lane
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, rbt@rbt.ca, agentm@cmu.edu
Subject: Re: Column limits in table/ views
In-Reply-To: <17237.1055168543@sss.pgh.pa.us>
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