Re: Dept of ugly hacks: eliminating padding space in system indexes

I would mention in the C comment that we are doing this for space
savings, but other than that, it seems fine.

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Tom Lane wrote:
> I was thinking a bit about how we pad columns of type NAME to
> fixed-width, even though they're semantically equivalent to C strings.
> The reason for wasting that space is that it makes it possible to
> overlay a C struct onto the leading columns of most system catalogs.
> I don't wish to propose changing that (at least not today), but it
> struck me that there is no reason to overlay a C struct onto index
> entries, and that getting rid of the padding space would be even more
> useful in an index than in the catalog itself. It turns out to be
> dead easy to implement this: effectively, we just decree that the
> index column storage type for NAME is always CSTRING. Because the
> two types are effectively binary-compatible as long as you don't
> look at the padding, the attached ugly-but-impressively-short patch
> seems to accomplish this. It passes the regression tests anyway.
> Here are some numbers about the space savings in a virgin database:
>
> CVS HEAD w/patch savings
>
> pg_database_size('postgres') 4439752 4071112 8.3%
> pg_relation_size('pg_class_relname_nsp_index') 57344 40960 28%
> pg_relation_size('pg_proc_proname_args_nsp_index') 319488 204800 35%
>
> Cutting a third off the size of a system index has got to be worth
> something, but is it worth a hack as ugly as this one?
>
> regards, tom lane
>
>

Content-Description: index-name-as-cstring.patch

> Index: src/backend/catalog/index.c
> ===================================================================
> RCS file: /cvsroot/pgsql/src/backend/catalog/index.c,v
> retrieving revision 1.300
> diff -c -r1.300 index.c
> *** src/backend/catalog/index.c 19 Jun 2008 00:46:04 -0000 1.300
> --- src/backend/catalog/index.c 23 Jun 2008 19:34:54 -0000
> ***************
> *** 262,267 ****
> --- 262,278 ----
>
> ReleaseSysCache(tuple);
> }
> +
> + /*
> + * For an index on NAME, force the index storage to be CSTRING,
> + * rather than padded to fixed length.
> + */
> + if (to->atttypid == NAMEOID)
> + {
> + to->atttypid = CSTRINGOID;
> + to->attlen = -2;
> + to->attalign = 'c';
> + }
> }
>
> return indexTupDesc;

>
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--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +