From: | John Naylor <jcnaylor(at)gmail(dot)com> |
---|---|
To: | Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> |
Cc: | "Nasby, Jim" <nasbyj(at)amazon(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: PostgreSQL Limits and lack of documentation about them. |
Date: | 2018-11-06 10:47:37 |
Message-ID: | CAJVSVGVOme8JgZ17mFrhZuJX=mfgr4AxzpeatoP87uappAmMLA@mail.gmail.com |
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Lists: | pgsql-hackers |
On 11/1/18, Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> wrote:
> So (with 8k blocks) the limit on the number of non-null external-toasted
> columns is about 450, while you can have the full 1600 columns if they
> are integers or smaller, or just over 1015 bigints. But you can have
> 1600 text columns if they average 4 bytes or less (excluding length
> byte).
>
> If you push too close to the limit, it may even be possible to overflow
> the tuple size by setting fields to null, since the null bitmap is only
> present if at least one field is null. So you can have 1010 non-null
> bigints, but if you try and do 1009 non-null bigints and one null, it
> won't fit (and nor will 999 non-nulls and 11 nulls, if I calculated
> right).
Thanks for that, Andrew, that was insightful. I drilled down to get
the exact values:
Non-nullable columns:
text (4 bytes each or less): 1600
toasted text: 452
int: 1600
bigint: 1017
Nullable columns with one null value:
text (4 bytes each or less): 1600
toasted text: 449
int: 1600
bigint: 1002
-John Naylor
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