On Thu, Aug 20, 2015 at 04:07:36PM -0700, Peter Geoghegan wrote:
> On Sat, Jun 13, 2015 at 3:53 PM, Peter Geoghegan <p...@heroku.com> wrote:
> > I think we should really address this. Attached patch adds a new
> > release note item for it. It also adds to the documentation that
> > explains why users should prefer varchar(n)/text to character(n); the
> > lack of abbreviated key support now becomes a huge disadvantage for
> > character(n), whereas in previous versions the disadvantages were
> > fairly minor.
> >
> > In passing, I updated the existing sort item to reflect that only
> > varchar(n), text, and numeric benefit from the abbreviation
> > optimization (not character types more generally + numeric), and added
> > a note on the effectiveness of the abbreviation optimization alone.
>
> A recent e-mail from Kaigai-san [1] reminded me of this item. I really
> think this limitation of char(n) needs to be documented along the
> lines I proposed here back in June. Benchmarks like TPC-H use char(n)
> extensively, since it's faster in other systems. However, PostgreSQL
> now has hugely inferior sort performance for that type as compared to
> text/varchar(n). This needs to be highlighted.
>
> [1]
> http://www.postgresql.org/message-id/flat/CAM3SWZRRCs6KAyN-bDsh0_pG=8xm3fvcf1x9dlsvd3wvbt1...@mail.gmail.com#CAM3SWZRRCs6KAyN-bDsh0_pG=8xm3fvcf1x9dlsvd3wvbt1...@mail.gmail.com
I have applied the attached patch to document this in the data type docs.
--
Bruce Momjian <br...@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ Everyone has their own god. +
diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
new file mode 100644
index 8113ddf..4d883ec
*** a/doc/src/sgml/datatype.sgml
--- b/doc/src/sgml/datatype.sgml
*************** SELECT '52093.89'::money::numeric::float
*** 1139,1145 ****
advantages in some other database systems, there is no such advantage in
<productname>PostgreSQL</productname>; in fact
<type>character(<replaceable>n</>)</type> is usually the slowest of
! the three because of its additional storage costs. In most situations
<type>text</type> or <type>character varying</type> should be used
instead.
</para>
--- 1139,1146 ----
advantages in some other database systems, there is no such advantage in
<productname>PostgreSQL</productname>; in fact
<type>character(<replaceable>n</>)</type> is usually the slowest of
! the three because of its additional storage costs and slower
! sorting. In most situations
<type>text</type> or <type>character varying</type> should be used
instead.
</para>
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