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RE: full-scan vs index for "small" tables

From: Cary Millsap <>
Date: Tue, 27 Jun 2006 23:24:22 -0500
Message-ID: <>

A rough histogram of what I've seen in the past ten years is this:  

40% - inefficient SQL that hits the buffer cache ridiculously often for the same blocks over and over

40% - application 3GL code that parses inside of loops, processes one row at a time, etc.

5% - slow or too-frequent disk I/O

15% - miscellaneous other (includes configuration errors, enqueue problems, bbw problems, etc.)  

Many instances of the first two problem types listed here are invisible to system-wide monitoring tools, which, I think, is why we see them so much.    

Cary Millsap

Hotsos Enterprises, Ltd.

Nullius in verba  

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From: Steve Perry [] Sent: Tuesday, June 27, 2006 9:47 PM
To: Cary Millsap
Cc: Mladen Gogala; Subject: Re: full-scan vs index for "small" tables  

On Jun 27, 2006, at 09:52 AM, Cary Millsap wrote:  

"Most people guess "It's got to be I/O." But "it" is I/O in fewer than 5% of cases I've witnessed since about 1995"    

What have been the majority of problems you've run into?  

For me, IO used to be the problem ( after 2GB drives went away and before SANs showed up). Mainly because the sysadmins treated database file systems like "file servers". they'd put everything on a few spindles and fill them up. then mgt. would say the servers were under utilized if the average utilization was below 60% so they would load up the servers with applications to max out the memory.

After I changed companies and started using SANs, most problems seem to be cpu bound on the server. I've seen a few really good nested loops queries on small tables that would take hours to complete.  

I'm curious what others have run into.    

Received on Tue Jun 27 2006 - 23:24:22 CDT

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