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.
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.
The Script
Consider the following minimally complete and verifiable example code that can be used to reproduce the problem:
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SET NOCOUNT ON; USE master; GO --Create a new, blank database for our test IF EXISTS (SELECT 1 FROM sys.databases d WHERE d.name = 'test_stats_alter') BEGIN ALTER DATABASE test_stats_alter SET SINGLE_USER WITH ROLLBACK IMMEDIATE; DROP DATABASE test_stats_alter; END CREATE DATABASE test_stats_alter; ALTER DATABASE test_stats_alter SET AUTO_CREATE_STATISTICS OFF; ALTER DATABASE test_stats_alter SET AUTO_UPDATE_STATISTICS OFF; GO USE test_stats_alter; GO CREATE TABLE dbo.stats_test ( id int NOT NULL IDENTITY(1,1) PRIMARY KEY CLUSTERED , d varchar(30) NOT NULL ); CREATE TABLE dbo.dates ( d varchar(30) NOT NULL ) GO --Insert a bunch of rows to allow the query optimizer to perform actual work. INSERT INTO dbo.stats_test (d) SELECT CONVERT(datetime, DATEADD(DAY, CONVERT(int, CRYPT_GEN_RANDOM(2)), '1900-01-01T00:00:00')) FROM sys.syscolumns sc1 CROSS JOIN sys.syscolumns sc2 INSERT INTO dbo.dates (d) SELECT CONVERT(datetime, DATEADD(DAY, CONVERT(int, CRYPT_GEN_RANDOM(2)), '1900-01-01T00:00:00')) FROM sys.syscolumns sc1 GO --Manually create a stats object CREATE STATISTICS stats_test_st1 ON dbo.stats_test(d) WITH FULLSCAN, NORECOMPUTE; GO --Attempt to alter the column with the manual stats object defined. --This will fail with Msg 5074, Level 16, State 1, Line xx ALTER TABLE dbo.stats_test ALTER COLUMN d nvarchar(30) NOT NULL; GO |
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:
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--drop the stats object DROP STATISTICS dbo.stats_test.stats_test_st1; GO --Allow SQL Server to automatically create statistics ALTER DATABASE test_stats_alter SET AUTO_CREATE_STATISTICS ON; ALTER DATABASE test_stats_alter SET AUTO_UPDATE_STATISTICS ON; GO --Coerce SQL Server into automatically creating a stats object. --This is a complex enough query that SQL Server recognizes a --stats object would be helpful for good performance. SELECT st.id , st.d INTO dbo.stats_test_output FROM dbo.stats_test st LEFT JOIN dbo.dates d ON st.d = d.d WHERE st.d > '2017-06-01T00:00:00'; GO --See if SQL Server in fact created an auto-stats object on --the column. SELECT * FROM sys.stats st INNER JOIN sys.objects o ON st.object_id = o.object_id WHERE o.name = 'stats_test'; GO |
╔════════════╦════════════════════════════════╦══════════════╗ ║ name ║ name ║ auto_created ║ ╠════════════╬════════════════════════════════╬══════════════╣ ║ stats_test ║ PK__stats_te__3213E83FF58F8430 ║ 0 ║ ║ stats_test ║ _WA_Sys_00000002_21B6055D ║ 1 ║ ╚════════════╩════════════════════════════════╩══════════════╝
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--attempt to alter the table, which succeeds. ALTER TABLE dbo.stats_test ALTER COLUMN d nvarchar(30) NOT NULL; GO --check to see if the auto-created stats object still exists SELECT * FROM sys.stats st INNER JOIN sys.objects o ON st.object_id = o.object_id WHERE o.name = 'stats_test'; |
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.
[…] Hannah Vernon demonstrates that custom statistics and prevent you for modifying a column: […]