Thread

[HACKERS] Max size of data types and tuples. (fwd)

Bruce Momjian <maillist@candle.pha.pa.us> — 1998-01-10T03:08:29Z

Here's a blast from the past.  Shows how I keep those open issues in my
mailbox.

Forwarded message:
> From scrappy@postgreSQL.org Wed Jan 29 14:49:26 1997
> X-Authentication-Warning: hub.org: pgsql set sender to owner-pg95-dev@postgreSQL.org using -f
> Date: Wed, 29 Jan 1997 13:38:10 -0500
> From: aixssd!darrenk@abs.net (Darren King)
> Message-Id: <9701291838.AA22296@ceodev>
> To: hackers@postgreSQL.org
> Subject: [HACKERS] Max size of data types and tuples.
> Mime-Version: 1.0
> Content-Type: text/plain; charset=US-ASCII
> Content-Transfer-Encoding: 7bit
> Content-Md5: 9bBKVeeTrq97EhvkS6qT3A==
> Sender: owner-pg95-dev@hub.org
> Reply-To: hackers@hub.org, aixssd!darrenk@abs.net (Darren King)
> 
>  
> Does anyone know of a valid reason for the 4096 byte limit
> on the textual fields (char, varchar, text, etc...)?
> 
> Could it be bumped to (MAXTUPLEN - sizeof(int)) in the parser
> and in utils/adt/varchar.c?  MAXTUPLEN would have to be
> calculated more accurately though as noted in the comment
> around its definition.
> 
> 
> The following are just some of my observations and comments
> on things that I think should be cleaned up a little in
> the source (and I'll volunteer to do them).
> 
> 1.  Clean up the #defines for the max block size.  Currently,
> there are at least four references to 8192...
> 
> include/config.h
> include/storage/bufmgr.h
> include/optimizer/internal.h
> backend/rewrite/rewriteDefine.c
> 
> The _PAGE_SIZE_ in internal.h is _not_ used anywhere in the
> optimizer.  This define should be linked to BLCKSZ, but would
> be better to remove it and just use BLCKSZ.
> 
> optimizer/costsize.c includes storage/bufpage.h for BLCKSZ,
> but in fact this is defined in config.h!.  Also included in
> 
> executor/nodeHash.c
> executor/nodeHashjoin.c
> utils/sort/psort.c
> 
> __These includes of storage/bufpage.h can be removed.__
> 
> 
> There should be      #define MAX_BLOCK_SIZE       8192
>                      #define CURRENT_BLOCK_SIZE   8192
> 
> The MAX_BLOCK_SIZE is a hard and fast limit since only 13 bits
> can be used in offsets and the like.  I believe that in the
> future, PostgreSql should support block sizes other than 8k,
> like 2k and 4k.  Long-term goal, but the code should be done
> to allow this at a future time.  The comments in storage/bufpage.h
> suggest to me that the original designers of Postgres had this
> in mind.
> 
> 
> 2.  Once the block size issue is taken care of, calculate the
> maximum tuple size more accurately.
> 
> 
> 3.  When #1 & #2 are resolved, let the textual fields have a max
> of (MAX_TUPLE_SIZE - sizeof(int)).
> 
> 
> 4.  Since only 13 bits are needed for storing the size of these
> textual fields in a tuple, could PostgreSql use a 16-bit int to
> store it?  Currently, the size is padded to four bytes in the
> tuple and this eats space if you have many textual fields.
> Without further digging, I'm assuming that the size is double-word
> aligned so that the actual text starts on a double-word boundary.
> 
> Thanks for reading this far. :)
> 
> Comments, suggestions most welcome...
> 
> 
> Darren    darrenk@insightdist.com
> 


-- 
Bruce Momjian
maillist@candle.pha.pa.us