ALTER TABLE ALTER COLUMN failed because one or more objects access this column

There are a variety of causes for the message shown in the title of this post. Manually creating statistics for a column is one such cause. This post shows how that works, and what you need to do to get around the error. Columns that have manually-created statistics attached cannot have their properties modified without first dropping the stats object – this is to ensure the stats object accurately reflects the content of the column. SQL Server returns an error message stating “ALTER TABLE ALTER COLUMN failed because one or more objects access this column.” I ran into this limitation recently when attempting to modify the datatype for a column from varchar to nvarchar. This database has auto create statistics disabled. As a result, manually creating statistics objects is critical to ensuring good query performance.

alter table alter column failed!

     Have a failed column or three!

The Error Message

When attempting to modify a column that has a manually created statistics object attached, you receive the following “ALTER TABLE ALTER COLUMN failed” error message:

Msg 5074, Level 16, State 1, Line 36
The statistics '<name>' is dependent on column '<col>'.
Msg 4922, Level 16, State 9, Line 36
ALTER TABLE ALTER COLUMN <col> failed because one or more objects access this column.

Interestingly, if SQL Server has auto-created a stats object on a column, and you subsequently modify that column, you receive no such error. SQL Server silently drops the statistics object, and modifies the column. The auto-created stats object is not automatically recreated until a query is executed that needs the stats object. This difference in how auto-created stats and manually created stats are treated by the engine can make for some confusion.

Ads by Google, Paying the Rent:

The Script

Consider the following minimally complete and verifiable example code that can be used to reproduce the problem:

SQL Server returns this error:

Msg 5074, Level 16, State 1, Line 47
The statistics 'stats_test_st1' is dependent on column 'd'.
Msg 4922, Level 16, State 9, Line 47
ALTER TABLE ALTER COLUMN d failed because one or more objects access this column.

Let’s continue on:

╔════════════╦════════════════════════════════╦══════════════╗
║    name    ║              name              ║ auto_created ║
╠════════════╬════════════════════════════════╬══════════════╣
║ stats_test ║ PK__stats_te__3213E83FF58F8430 ║            0 ║
║ stats_test ║ _WA_Sys_00000002_21B6055D      ║            1 ║
╚════════════╩════════════════════════════════╩══════════════╝

The auto-created stats object has been silently dropped:

╔════════════╦════════════════════════════════╦══════════════╗
║    name    ║              name              ║ auto_created ║
╠════════════╬════════════════════════════════╬══════════════╣
║ stats_test ║ PK__stats_te__3213E83FF58F8430 ║            0 ║
╚════════════╩════════════════════════════════╩══════════════╝

In Summary

Manually adding statistics objects can be a blessing for performance, however you need to recognize the limitations this creates for future object modifications.

If you have auto_create_statistics turned off, you probably want to update your stats objects on a regular basis to ensure good performance. See my statistics update job for details about how to do that.

Read the other articles in our series on SQL Server Internals.

Ads by Google, Paying the Rent: