Deadlock when querying INFORMATION_SCHEMA

Question

I have a process which dynamically alters my SQL2K5 table structure according to changes in a published meta-data layer.

For example, if a new column needs to be added and the table has NO dependancies - the steps would be: 1. Create scripts using T-SQL for any indexes & primary keys that already exist on the table [these scripts are included below] 2. Drop the table 3. Re-create the table from the meta-layer that has the new column 4. Execute the scripts created in step#1 5. Populate the table using BulkCopy

The above is initiated via a .NET assembly and runs in 3 concurrent streams on a daily basis.

I am receiving a deadlock error in step #1 - when I access the INFORMATION_SCHEMA tables to script out the indexes/keys. I have used the hint WITH(NOLOCK) in these scripts thinking this should prevent any locking when 3 streams of these actions are running concurrently. A table can only be processed (the create or scripting) in 1 stream.

Is there something more I need to do???

Any comments greatly appreciated.

[Scripts]

ALTER Procedure [dbo].[s$spScriptPrimaryKeyForTable]
@Tablename varchar(100)
AS 


-- Get all existing primary keys
DECLARE cPK CURSOR FOR
SELECT TABLE_NAME, CONSTRAINT_NAME 
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WITH(NOLOCK)
WHERE upper(TABLE_NAME)=upper(@Tablename)
ORDER BY TABLE_NAME

DECLARE @PkTable SYSNAME
DECLARE @PkName SYSNAME

-- Loop through all the primary keys
OPEN cPK
FETCH NEXT FROM cPK INTO @PkTable, @PkName
WHILE (@@FETCH_STATUS = 0)
BEGIN
DECLARE @PKSQL NVARCHAR(4000) SET @PKSQL = ''
SET @PKSQL = 'ALTER TABLE ' + @PkTable + ' ADD CONSTRAINT ' + @PkName + ' PRIMARY KEY CLUSTERED ('

-- Get all columns for the current primary key
DECLARE cPKColumn CURSOR FOR
SELECT COLUMN_NAME 
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE WITH(NOLOCK)
WHERE TABLE_NAME = @PkTable AND CONSTRAINT_NAME = @PkName
ORDER BY ORDINAL_POSITION
OPEN cPKColumn

DECLARE @PkColumn SYSNAME
DECLARE @PkFirstColumn BIT SET @PkFirstColumn = 1
-- Loop through all columns and append the sql statement
FETCH NEXT FROM cPKColumn INTO @PkColumn
WHILE (@@FETCH_STATUS = 0)
BEGIN
IF (@PkFirstColumn = 1)
SET @PkFirstColumn = 0
ELSE
SET @PKSQL = @PKSQL + ', '

SET @PKSQL = @PKSQL + @PkColumn

FETCH NEXT FROM cPKColumn INTO @PkColumn
END
CLOSE cPKColumn
DEALLOCATE cPKColumn

SET @PKSQL = @PKSQL + ')'
-- Print the primary key statement
-- PRINT @PKSQL

FETCH NEXT FROM cPK INTO @PkTable, @PkName
END
CLOSE cPK
DEALLOCATE cPK


SELECT ISNULL(@PKSQL,' ')

================

ALTER Procedure [dbo].[s$spScriptIndexesForTable]
@Tablename varchar(100)

AS 

DECLARE @RetVal varchar(4000)
SET @RetVal = ''

-- Get all existing indexes, but NOT the primary keys 
DECLARE cIX CURSOR FOR 
SELECT OBJECT_NAME(SI.Object_ID), SI.Object_ID, SI.Name, SI.Index_ID 
FROM Sys.Indexes SI WITH(NOLOCK)
LEFT JOIN INFORMATION_SCHEMA.TABLE_CONSTRAINTS TC WITH(NOLOCK) ON SI.Name = TC.CONSTRAINT_NAME AND OBJECT_NAME(SI.Object_ID) = TC.TABLE_NAME 
WHERE TC.CONSTRAINT_NAME IS NULL 
AND OBJECTPROPERTY(SI.Object_ID, 'IsUserTable') = 1 
AND upper(OBJECT_NAME(SI.Object_ID))=upper(@Tablename)
ORDER BY OBJECT_NAME(SI.Object_ID), SI.Index_ID 

DECLARE @IxTable SYSNAME 
DECLARE @IxTableID INT 
DECLARE @IxName SYSNAME 
DECLARE @IxID INT 

-- Loop through all indexes 
OPEN cIX 
FETCH NEXT FROM cIX INTO @IxTable, @IxTableID, @IxName, @IxID 
WHILE (@@FETCH_STATUS = 0) 
BEGIN 
DECLARE @IXSQL NVARCHAR(4000) 
--SET @PKSQL = '' 
SET @IXSQL = 'CREATE ' 

-- Check if the index is unique 
IF (INDEXPROPERTY(@IxTableID, @IxName, 'IsUnique') = 1) 
SET @IXSQL = @IXSQL + 'UNIQUE ' 
-- Check if the index is clustered 
IF (INDEXPROPERTY(@IxTableID, @IxName, 'IsClustered') = 1) 
SET @IXSQL = @IXSQL + 'CLUSTERED ' 

SET @IXSQL = @IXSQL + 'INDEX ' + @IxName + ' ON [' + @IxTable + '] (' 

-- Get all columns of the index 
DECLARE cIxColumn CURSOR FOR 
SELECT SC.Name,IC.[is_included_column],IC.is_descending_key 
FROM Sys.Index_Columns IC WITH(NOLOCK)
JOIN Sys.Columns SC WITH(NOLOCK) ON IC.Object_ID = SC.Object_ID AND IC.Column_ID = SC.Column_ID 
WHERE IC.Object_ID = @IxTableID AND Index_ID = @IxID 
ORDER BY IC.Index_Column_ID,IC.is_included_column 

DECLARE @IxColumn SYSNAME 
DECLARE @IxIncl bit 
DECLARE @Desc bit 
DECLARE @IxIsIncl bit set @IxIsIncl = 0 
DECLARE @IxFirstColumn BIT SET @IxFirstColumn = 1 

-- Loop throug all columns of the index and append them to the CREATE statement 
OPEN cIxColumn 
FETCH NEXT FROM cIxColumn INTO @IxColumn, @IxIncl, @Desc 
WHILE (@@FETCH_STATUS = 0) 
BEGIN 

IF (@IxFirstColumn = 1) 
BEGIN 
SET @IxFirstColumn = 0 
END 
ELSE 
BEGIN 
--check to see if it's an included column 
IF ((@IxIsIncl = 0) AND (@IxIncl = 1)) 
BEGIN 
SET @IxIsIncl = 1 
SET @IXSQL = @IXSQL + ') INCLUDE (' 
END 
ELSE 
BEGIN 
SET @IXSQL = @IXSQL + ', ' 
END 
END 

SET @IXSQL = @IXSQL + '[' + @IxColumn + ']' 
--check to see if it's DESC 
IF @Desc = 1 
SET @IXSQL = @IXSQL + ' DESC' 

FETCH NEXT FROM cIxColumn INTO @IxColumn, @IxIncl, @Desc 
END 
CLOSE cIxColumn 
DEALLOCATE cIxColumn 

SET @IXSQL = @IXSQL + ')' 

-- Print out the CREATE statement for the index 
--SELECT 'IXSQL: ' + @IXSQL
IF @RetVal IS NULL
SET @RetVal = ''
--SELECT 'Retval: ' + @RetVal
SET @RetVal = @RetVal + @IXSQL + ' ' 

FETCH NEXT FROM cIX INTO @IxTable, @IxTableID, @IxName, @IxID 
END 

CLOSE cIX 
DEALLOCATE cIX 

SELECT ISNULL(@RetVal,' ')

Answer 1

1. INFORMATION_SCHEMA views are just that - views. You can't update them so they are unlikely to cause any deadlocks. If you want to determine the real source (which I assume has something to do with your alters, or other code within the cursor that you didn't show, or other code you're calling in combination with calling these procedures - since selects against views and then selecting variables can't be the cause), I suggest reading Gail Shaw's blog post on interpreting deadlocks.

2. In spite of (1) I still suggest using more modern catalog views than INFORMATION_SCHEMA. The same information can be derived from, for example, sys.key_constraints.

3. You're using the default cursor options; and you're nesting cursors. If you end up still using cursors, you should get in the habit of using a less resource intensive cursor (e.g. LOCAL STATIC FORWARD_ONLY READ_ONLY).

4. You don't actually need a cursor to do this. Here is how I would re-write the PK table script:

   CREATE PROCEDURE dbo.ScriptPKForTable
       @TableName SYSNAME
   AS
   BEGIN
       SET NOCOUNT ON;
   
       DECLARE 
         @pkName    SYSNAME,
         @clustered BIT,
         @object_id INT,
         @sql       NVARCHAR(MAX);
   
       SELECT
         @object_id = OBJECT_ID(UPPER(@TableName));
   
       SELECT
         @pkName = kc.name,
         @clustered = CASE i.[type] 
           WHEN 1 THEN 1 ELSE 0 END
       FROM 
           sys.key_constraints AS kc
       INNER JOIN 
           sys.indexes AS i
           ON kc.parent_object_id = i.[object_id]
           AND kc.unique_index_id = i.index_id
       WHERE
           kc.parent_object_id = @object_id
           AND kc.[type] = 'pk';
   
       SET @sql = N'ALTER TABLE ' + QUOTENAME(@TableName)
         + ' ADD CONSTRAINT ' + @pkName 
         + ' PRIMARY KEY ' + CASE @clustered 
         WHEN 1 THEN 'CLUSTERED' ELSE '' END + ' (';
   
       SELECT
         @sql = @sql + c.name + ','
       FROM 
         sys.index_columns AS ic
       INNER JOIN
         sys.indexes AS i 
         ON ic.index_id = i.index_id
         AND ic.[object_id] = i.[object_id]
       INNER JOIN 
         sys.key_constraints AS kc
         ON i.[object_id] = kc.[parent_object_id]
         AND kc.unique_index_id = i.index_id
       INNER JOIN 
         sys.columns AS c
         ON i.[object_id] = c.[object_id]
         AND ic.column_id = c.column_id
       WHERE
         kc.[type] = 'PK'
         AND kc.parent_object_id = @object_id
       ORDER BY key_ordinal;
   
       SET @sql = LEFT(@sql, LEN(@sql) - 1) + ');';
   
       SELECT COALESCE(@sql, ' ');
   END
   GO
   

As for the index creation script, I think there is a better way to do this (again without explicit cursors, not that avoiding the cursor is the goal, but the code is going to be a LOT cleaner). First you need a function to build either key or include columns from the index:

CREATE FUNCTION dbo.BuildIndexColumns
(
    @object_id        INT,
    @index_id         INT,
    @included_columns BIT
)
RETURNS NVARCHAR(MAX)
AS
BEGIN
  DECLARE @s NVARCHAR(MAX);

  SELECT @s = N'';

  SELECT @s = @s + c.name + CASE ic.is_descending_key
    WHEN 1 THEN ' DESC' ELSE '' END + ',' 
    FROM sys.index_columns AS ic
    INNER JOIN sys.columns AS c
    ON ic.[object_id] = c.[object_id]
    AND ic.column_id = c.column_id
    WHERE c.[object_id] = @object_id
    AND ic.[object_id] = @object_id
    AND ic.index_id = @index_id
    AND ic.is_included_column = @included_columns
    ORDER BY ic.key_ordinal;

  IF @s > N''
    SET @s = LEFT(@s, LEN(@s)-1);

  RETURN (NULLIF(@s, N''));
END
GO

With that function in place, a ScriptIndexes procedure is pretty easy:

CREATE PROCEDURE dbo.ScriptIndexesForTable
    @TableName SYSNAME
AS
BEGIN
  SET NOCOUNT ON;

  DECLARE
      @sql       NVARCHAR(MAX),
      @object_id INT;

  SELECT @sql = N'', @object_id = OBJECT_ID(UPPER(@TableName));

  SELECT @sql = @sql + 'CREATE '
      + CASE i.is_unique WHEN 1 THEN 'UNIQUE ' ELSE '' END
      + CASE i.[type] WHEN 1 THEN 'CLUSTERED ' ELSE '' END
      + ' INDEX ' + i.name + ' ON ' + QUOTENAME(@TableName) + ' (' 
      + dbo.BuildIndexColumns(@object_id, i.index_id, 0)
      + ')' + COALESCE(' INCLUDE(' 
      + dbo.BuildIndexColumns(@object_id, i.index_id, 1)
      + ')', '') + ';' + CHAR(13) + CHAR(10)
  FROM
      sys.indexes AS i
  WHERE
      i.[object_id] = @object_id
      -- since this will be covered by ScriptPKForTable:
      AND i.is_primary_key = 0
  ORDER BY i.index_id;

  SELECT COALESCE(@sql, ' ');
END
GO

Note that my solution does not assume the PK is clustered (your PK script hard-codes CLUSTERED but then your index script assumes that any of the indexes could be clustered). I also ignore additional properties such as filegroup, partitioning, or filtered indexes (not supported in 2005 anyway).