Sql-server – In what circumstances I am more likely to benefit from auto update stats async on

That was actually my question at AskSSC. I should have just tested it myself as I accepted an incorrect answer.

With the following test table

CREATE TABLE StatsTest
(
a varchar(max),
b varchar(max)
)

DECLARE @VCM VARCHAR(MAX) = 'A'

INSERT INTO StatsTest
SELECT TOP 20000
       REPLICATE(@VCM,10000),
       REPLICATE(@VCM,10000)
FROM master..spt_values v1,  master..spt_values v2   

And the following test code

         SqlConnection connection = new SqlConnection(...);
            connection.Open();

            SqlCommand command = connection.CreateCommand();
            command.CommandTimeout = 12;
            command.CommandType = CommandType.Text;
            command.CommandText = @"SELECT COUNT(*)
FROM StatsTest
WHERE 
      a LIKE '%foo%' OR 
      b LIKE '%foo%' ";
            command.ExecuteScalar();

Profiler shows the following

First it creates the stats for column b successfully (the initial SP:StmtStarting /SP:StmtCompleted pair) . Then it starts creating the stats for column a (The selected SP:StmtStarting entry in the screen shot). This entry is followed by an AUTOSTATS event confirming that the statistics on b were created then the timeout kicks in.

It can be seen that the stats creation occurs on the same spid as the query and so this also aborts the creation of stats on column a. At the end of the process only one set of stats exists on the table.

Edit

The above refers to stats creation, to test auto update of the stats I ran the above query without a timeout so both sets of stats were successfully created then updated all columns of all rows so that the stats would be out of date and re-ran the test. The trace for that is pretty similar

Finally just for completeness with SET AUTO_UPDATE_STATISTICS_ASYNC ON the trace looks as follows. It can be seen that system spids are used to perform the operation and they are unaffected by the query timeout as would be expected.

You have two questions:

1. Do you have to restart after setting auto update stats on? No.

2. Should you consider dropping the user created stats? If your statistics updates jobs (typically done with a maintenance plan or with Ola Hallengren's maintenance scripts) are taking longer than your maintenance window allows, then yes. SQL Server has to read the entire index (or table) to update each statistic, so you can end up with really long stats update jobs. If your maintenance window isn't a problem for you, then you can leave 'em as is.