Kaley's (mostly) Oracle Database Blog
Hi! I'm Kaley Crum, Oracle DBA.
Welcome to my (mostly) Oracle database blog.

Archive: Sep 2018

Summing Interval Data Types

Something I’ve occasionally thought would be really nice is if Oracle had a good way of summing up interval data types, much the same way that you can SUM() number data types.  Occasionally I’ve designed processes that record start and stop timestamps of operations.  If you want to know how long the operation lasted, you could do the end timestamp minus the start timestamp to get a DAY TO SECOND INTERVAL data type.

If I want to know the total runtime of a set of operations, there’s not a nice, slick, easy way to do this.

Usually, I get away with some jankedy code like this:

First, you gotta cast the timestamps as dates (meaning you lose any fractional-seconds).

Then you subtract the start date from the end date.  That gives you a number, which represents the number of days (possibly fractional).

Oracle then allows you to sum up those numbers.

If you want the number back in interval format, you can supply it to a NUMTODSINTERVAL() function, passing in ‘DAY’ as the second argument.

It sucks, it’s not smooth, it loses precision, the maintaining developer has to double-take to try to figure out what it is that I’m doing.  Wouldn’t it be nice if there was some smooth way to just add up intervals?

Well, after giving it some thought, I decided to create my own…

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Access and Filter – Is There an Echo in Here? (Why did Oracle Repeat the Same Filter Twice?)

There are lots of resources that will tell you about the “access” and “filter” lines you see in the Predicates section of your execution plan.  Most resources will tell you…”access is more efficient than filter.” And, generally, that’s true.  Access tells us where to go in our index (thereby eliminating rows before we reach them) whereas filter tells us once we’ve arrived at a row that “it’s no good and must be discarded.”

But you may have noticed times when there seems to be and echo in your plan, and it repeats the same predicate filter in both the FILTER and ACCESS section of your execution plan.  What then?  Why does Oracle need both?  Shouldn’t just one be enough?

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