Phone rings……..I answer…….a DBA from a third party vendor has supplied someone with two scripts. The first script is a simple insert with a where clause:
insert into [databasename].[dbo].[tablename]
( column1,column2, column3, column4 )
select column1, column2, column3, column4
First question I ask; how much data is loading into the table – answer millions of records; the vendor gave us a script to see if the number is increasing. What is the script; do you have it? Sure…..script below:
select COUNT(*) from tablename
I was reminded of something I came across several years ago about this very scenario so figured why not put it to the test. I will try to explain to the best of my ability why the the second query was taking an hour to run.
The first problem I see right off hand is the COUNT(*) statement has to do a table scan; it is a requirement to figure out the calculation to return the result set. You take this statement and run it against several million row tables with a ton of reads on it you have a recipe of being prepared to sit and watch and wait for the result set to return.
How Do I Get Around This?
It’s not that difficult and here is a nice trick to provide a quick solution. I’m a huge fan of DMV’s and it just so happens that you can utilize one to return row counts for all tables in a database or specific tables in the database:
FROM sys.indexes AS i
INNER JOIN sys.objects AS o ON i.OBJECT_ID = o.OBJECT_ID
INNER JOIN sys.dm_db_partition_stats AS ddps ON i.OBJECT_ID = ddps.OBJECT_ID
AND i.index_id = ddps.index_id
WHERE i.index_id < 2
AND o.is_ms_shipped = 0
–AND o.name = [table name] /*UNCOMMENT AND PLUG IN TABLE NAME FOR SPECIFIC TABLE INFO*/
ORDER BY o.NAME
The result will give you the specific table name with row count
Don’t be alarmed by using the system object. Unlike others the row count does not depend on any updated statistics so the count is accurate. On this 132 million record table I can get the result set to return immediately.
Next time you get stuck waiting on a COUNT(*) statement to run; think about using a DMV; for a listing check out what Microsoft has listed into categories
It is always nice to have some tricks up your sleeve; especially when dealing with outside vendors.