Question

Counting tables with large amount of data may be very slow, sometimes it takes minutes; it also may generate deadlock on a busy server. I want to display real values, NOLOCK is not an option.

The servers I use is SQL Server 2005 or 2008 Standard or Enterprise – if it matters.
I can imagine that SQL Server maintains the counts for every table and if there is no WHERE clause I could get that number pretty quickly, right?

For example:

SELECT COUNT(*) FROM myTable

should immediately return with the correct value. Do I need to rely on statistics to be updated?

Answer 1

Very close approximate (ignoring any in-flight transactions) would be:

SELECT SUM(p.rows) FROM sys.partitions AS p
  INNER JOIN sys.tables AS t
  ON p.[object_id] = t.[object_id]
  INNER JOIN sys.schemas AS s
  ON s.[schema_id] = t.[schema_id]
  WHERE t.name = N'myTable'
  AND s.name = N'dbo'
  AND p.index_id IN (0,1);

This will return much, much quicker than COUNT(*), and if your table is changing quickly enough, it’s not really any less accurate – if your table has changed between when you started your COUNT (and locks were taken) and when it was returned (when locks were released and all the waiting write transactions were now allowed to write to the table), is it that much more valuable? I don’t think so.

If you have some subset of the table you want to count (say, WHERE some_column IS NULL), you could create a filtered index on that column, and structure the where clause one way or the other, depending on whether it was the exception or the rule (so create the filtered index on the smaller set). So one of these two indexes:

CREATE INDEX IAmTheException ON dbo.table(some_column)
  WHERE some_column IS NULL;

CREATE INDEX IAmTheRule ON dbo.table(some_column)
  WHERE some_column IS NOT NULL;

Then you could get the count in a similar way using:

SELECT SUM(p.rows) FROM sys.partitions AS p
  INNER JOIN sys.tables AS t
  ON p.[object_id] = t.[object_id]
  INNER JOIN sys.schemas AS s
  ON s.[schema_id] = t.[schema_id]
  INNER JOIN sys.indexes AS i
  ON p.index_id = i.index_id
  WHERE t.name = N'myTable'
  AND s.name = N'dbo'
  AND i.name = N'IAmTheException' -- or N'IAmTheRule'
  AND p.index_id IN (0,1);

And if you want to know the opposite, you just subtract from the first query above.