Re: Alternative variable length structure - Mailing list pgsql-hackers

Jim C. Nasby wrote:
> On Wed, Jun 14, 2006 at 02:53:10PM -0400, Bruce Momjian wrote:
> > 
> > I assume the conclusion from this email thread is that though the idea
> > is interesting, the complexity added would not be worth the saving of a
> > few bytes.
>  
> Anyone do any testing?
> 
> I'm also wondering if this would be useful to allow fields larger than
> 1G.

The submitter showed the pathological case where a single char was
stored in a text field, and showed the reduced size (below).  There were
no performance numbers given.  It seems like an edge case, especially
since we have a "char" type that is a single byte.

---------------------------------------------------------------------------


> > 
> > ITAGAKI Takahiro wrote:
> > > Hi Hackers,
> > > 
> > > PostgreSQL can treat variable-length data flexibly, but therefore
> > > it consumes more spaces if we store short data. Headers of
> > > variable-length types use 4 bytes regardless of the data length.
> > > 
> > > My idea is to change the header itself to variable-length. 
> > > In order to reduce the size of short data, I wrote a patch to encode
> > > lengths into the first several bits of structure. Also, the alignments
> > > of the types were changed to 'char' from 'int'.
> > > 
> > > 
> > > I know my patch is still insufficient, for example, the types cannot
> > > be TOASTed. But I guess this compression works well for short text.
> > > 
> > > I'll appreciate any comments.
> > > thanks.
> > > 
> > > 
> > > ---- the result of patch ----
> > > 
> > > # create table txttbl (v1 text, v2 text, v3 text, v4 text);
> > > # create table strtbl (v1 string, v2 string, v3 string, v4 string);
> > > 
> > > # insert into txttbl values('A', 'B', 'C', 'D');
> > > # insert into strtbl values('A', 'B', 'C', 'D');
> > > 
> > > # select * from pgstattuple('txttbl');
> > > -[ RECORD 1 ]------+------
> > > table_len          | 8192
> > > tuple_count        | 1
> > > tuple_len          | 57    <-- 28 + (5+3) + (5+3) + (5+3) + (5)
> > > ...
> > > 
> > > # select * from pgstattuple('strtbl');
> > > -[ RECORD 1 ]------+------
> > > table_len          | 8192
> > > tuple_count        | 1
> > > tuple_len          | 36    <-- 28 + 2 + 2 + 2 + 2
> > > ...
> > > 
> > > ---
> > > ITAGAKI Takahiro
> > > NTT Cyber Space Laboratories
> > 
> > [ Attachment, skipping... ]
> > 
> > > 
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> > 
> > -- 
> >   Bruce Momjian   http://candle.pha.pa.us
> >   EnterpriseDB    http://www.enterprisedb.com
> > 
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> 
> -- 
> Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
> Pervasive Software      http://pervasive.com    work: 512-231-6117
> vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461
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--  Bruce Momjian   http://candle.pha.pa.us EnterpriseDB    http://www.enterprisedb.com
 + If your life is a hard drive, Christ can be your backup. +