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Re: How can fixed and variable width columns perform similarly?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: sid(at)etsy(dot)com
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: How can fixed and variable width columns perform similarly?
Date: 2007-04-27 17:12:47
Message-ID: 16453.1177693967@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
"Siddharth Anand" <sid(at)etsy(dot)com> writes:
> My question wasn't phrased clearly. Oracle exhibits a performance
> degradation for very large-sized fields (CLOB types that I equate to
> PostGres' text type) when compared with the performance of field types
> like varchar that handle a max character limit of a few thousand bytes in
> Oracle.

> It sounds like PostGres doesn't exhibit this same difference. I wanted to
> understand how this could be and whether there was a trade-off.

Ah.  Well, the answer is that we change behavior dynamically depending
on the size of the particular field value, instead of hard-wiring it to
the declared column type.  It sounds like Oracle's CLOB might be doing
about the same thing as an out-of-line "toasted" field value in
Postgres.  In PG, text and varchar behave identically except that
varchar(N) adds an insert-time check on the length of the field value
--- but this is just a constraint check and doesn't have any direct
influence on how the value is stored.

			regards, tom lane

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