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Archive for January 25th, 2010

Counting concurrent sessions

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Answering questions asked on the site.

Steve asks:

I am trying to query a log table for a web service application and determine how many concurrent sessions are in progress at each moment a transaction is executed, based on a start date and an elapsed time for each command executed through the web service. (These metrics are logged after the fact, I'm trying to write daily performance reporting for the site).

Here's a simplified view of my base table design:

CREATE TABLE CONNECTIONS 
(
USERID VARCHAR2(30), 
HANDLE VARCHAR2(40), 
PROCESS_DATE DATE, 
COMMAND NUMBER(6,0), 
COUNT NUMBER(10,0), 
ELAPSED_TIME NUMBER(10,0), 
NUM_OF_RECS NUMBER(10,0), 
COMMAND_TIMESTAMP TIMESTAMP (6)
)

The question is: at there moment each transaction started, how many other transactions were active?

At each given moment, there is some number of active transaction. A transaction is active if the transaction begins before that moment and ends after it. This means that the moment should fall between command_timestamp and command_timestamp + elapsed_time / 86400000.

Database B-Tree indexes are not very good in queries that involve searching for a constant between two columns, so a self-join on the condition described above would be possible but not very efficient.

But these is a more simple solution.

Whenever a transaction starts, it increments the count of the open transactions. Whenever the transaction ends, it decrements it.

So we just can build a table of events: starts and ends of the transactions, ordered chronologically. Each start would be denoted with a +1, and each end with a -1. Then we should just calculate the number of the transactions open so far and subtract the number of the transactions closed.

This can be easily done merely by calculating the partial sum of these +1's and -1's, which is an easy task for Oracle's analytic functions.

Let's create a sample table. I'll put only the relevant columns there and add a stuffing column that would emulate actual payload, to measure performance:
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Written by Quassnoi

January 25th, 2010 at 11:00 pm

Posted in Oracle