Re: Performance differences using varchar, char and text

Yusuf wrote:
> In the Postgres 7.3.3. User Guide section 5.3, it says that there's no
> performance difference between the three type.  But in 'PostgreSQL
> Database Performance Tuning' by Jean-Paul ARGUDO (section 5: use correct
> datatype), it says there is a performance difference because of you
> might need more I/O to read the data (which makes sense).
>
> So, is there a performance difference?

Yusuf,

Here is what I noticed - I am using Postgres 7.3.2 (on a Sun box, not
sure what OS version) with the latest ODBC driver on a Windows XP Pro
box. I am in the process of conversion of a legacy VB app from using
Access 97 to PostgreSQL via an ODBC connection (DSN-less).

I had in my Access DB several tables which utilized Memo-type fields to
store data in a "multi-value" delimited format (will never do that
again). I tried to first convert them to TEXT type fields on the
PostgreSQL tables. These didn't work right (more on that later), so I
converted them to large VARCHAR field (ie, VARCHAR(100000), and larger,
in some cases).

I noticed when doing selects (via the ODBC driver) that any accesses to
these fields caused MASSIVE slowdowns on the select - whether I was
selecting for them, or if the field was part of the WHERE clause of the
SQL statement. I found that if I decreased the size of the field, the
speed would increase.

One thing I did note was that in using psql, selects on these fields
didn't matter - they seemed fast. I tend to think it was the ODBC driver
in some manner.

Later, I determined that the reason I was having troubles with the TEXT
fields was because I was using DAO. Switching the code to use ADO calls
instead fixed the issue, and I switched to using the TEXT type on the
fields. However, by then I had installed many "workarounds" to avoid
those fields as much as possible in my code that I don't know if the
selects on them would be the same, faster, or slower...

Hope this helps a little...

Andrew

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