From: | "Jaime Casanova" <systemguards(at)gmail(dot)com> |
---|---|
To: | "Craig A(dot) James" <cjames(at)modgraph-usa(dot)com> |
Cc: | pgsql-performance(at)postgresql(dot)org |
Subject: | Re: update == delete + insert? |
Date: | 2006-03-21 01:38:15 |
Message-ID: | c2d9e70e0603201738s6664b1fci1717188cce834a39@mail.gmail.com |
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Thread: | |
Lists: | pgsql-performance |
On 3/20/06, Craig A. James <cjames(at)modgraph-usa(dot)com> wrote:
> I've seen it said here several times that "update == delete + insert". On the other hand, I've noticed that "alter table [add|drop] column ..." is remarkably fast, even for very large tables, which leads me to wonder whether each column's contents are in a file specifically for that column.
>
> My question: Suppose I have a very "wide" set of data, say 100 columns, and one of those columns will be updated often, but the others are fairly static. I have two choices:
>
> Design 1:
> create table a (
> id integer,
> frequently_updated integer);
>
> create table b(
> id integer,
> infrequently_updated_1 integer,
> infrequently_updated_2 integer,
> infrequently_updated_3 integer,
> ... etc.
> infrequently_updated_99 integer);
>
> Design 2:
> create table c(
> id integer,
> frequently_updated integer,
> infrequently_updated_1 integer,
> infrequently_updated_2 integer,
> infrequently_updated_3 integer,
> ... etc.
> infrequently_updated_99 integer);
>
> If "update == delete + insert" is strictly true, then "Design 2" would be poor since 99 columns would be moved around with each update. But if columns are actually stored in separate files, the Designs 1 and 2 would be essentially equivalent when it comes to vacuuming.
>
> Thanks,
> Craig
>
design 1 is normalized and better
design 2 is denormalized and a bad approach no matter the RDBMS
update does delete + insert, and vacuum is the way to recover the space
--
Atentamente,
Jaime Casanova
"What they (MySQL) lose in usability, they gain back in benchmarks, and that's
all that matters: getting the wrong answer really fast."
Randal L. Schwartz
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