Common 'sequential read' misconception

Common 'sequential read' misconception

I am using Oracle 10G on an ECC6. When I look into SM50 there are many queries that are performing sequential reads. One of my colleagues told me that "sequential reads" means that the query is not using "Index" to scan. How can I resolve this issue and get an expensive SQL statement in the system?

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It is a very common misconception that "sequential reads in SM50 means that a full table scan is executed and an index is not used". The correct definition/concept is:

Direct read
The term "Direct read" refers to accesses in which a maximum of one line is returned by the database. This includes summarization queries such as SELECT COUNT and fully-qualified primary key accesses.

Sequential Read
A "Sequential Read" refers to all other read database accesses, in which there may be more than one line returned. It is true that the term "Sequential read" is a little deceptive since it implies that blocks are always read sequentially (as in a full table scan). In reality, however, when you perform a trace on these queries you will see that index is used in most cases.

So, "Direct read" and "Sequential read" in SM50 are worthless for a performance analysis. It only shows that the processes are running and processing one of the queries directed at the database interface.

For expensive SQL statements analysis, "Reads/User Calls" value (TCODE BACOCKPIT or ST04OLD) is an indication. If this value is above 15, the SQL statements should be checked in detail.

This was first published in January 2008