How to Hide Metadata Information Using Extended Properties

Ok, a more honest title for this post might be, 'How to Clean Up the Mess After You've Hosed the Extended Properties on Primary Key Constraints', but that wouldn't fit.

What I was trying to accomplish is a topic for another post. But while grumbling loudly while writing the code to fix what I had inadvertently done, I realized it could be used to "hide" information about tables. I say "hide", in quotes, because anyone with access to Books Online could find it easily. This is really just a way of storing information about tables, and not having it appear in SQL Server Management Studio (SSMS).

What I ran into involves the difference between primary key constraints and primary key indexes. The most important distinction between them is "existence"; there's no such thing as a primary key index. If you're confused, keep reading. If you've heard this story already, skip ahead a bit. Simply put, when someone is talking about "primary key indexes", they're using verbal shorthand for, "the index that the primary key constraint uses to enforce uniqueness." (As far as I know, this kind of index has no specific name, which is good, because if if did, we'd all confuse it with "primary key constraint", so we'd be in the same boat we are now, but without the ability to abruptly clarify things by saying, "but there's no such thing as a whatever-it-is." We should count our blessings.) To sum up, when it comes to primary keys, we have a "primary key constraint", and a unique index of some sort to help it. (We won't be getting into the whole clustered vs. non-clustered thing here.)

Now we're ready to talk about setting extended properties on primary key constraints. First we create a little test table:

CREATE TABLE MyTable
(
    MyPkColumn INT NOT NULL,
    CONSTRAINT PK_MyTable PRIMARY KEY (MyPkColumn) 
) 

And now we add an extended property to the PK_MyTable constraint, as so:

EXEC sys.sp_addextendedproperty 
    @name       = N'Extended property for PK_MyTable constraint'
  , @value      = N'This is the PK for MyTable'
  , @level0type = N'SCHEMA'
  , @level0name = N'dbo'
  , @level1type = N'TABLE'
  , @level1name = N'MyTable'
  , @level2type = N'CONSTRAINT'
  , @level2name = N'PK_MyTable'

Once added, this extended property will appear in SSMS, as expected. Oddly, it appears on the index-the-primary-key-constraint-uses, not on the constraint itself. (And no, I'm not suggesting you create tables in the master database.) So far, so good. But given that the "slot" for the index is already taken, and knowing what we do about about the non-existence of "primary key indexes", what happens if we do this?

EXEC sys.sp_addextendedproperty 
    @name       = N'Secret extended property for PK_MyTable index'
  , @value      = N'This is the PK index for MyTable'
  , @level0type = N'SCHEMA'
  , @level0name = N'dbo'
  , @level1type = N'TABLE'
  , @level1name = N'MyTable'
  , @level2type = N'INDEX'
  , @level2name = N'PK_MyTable'

Ideally (I think anyway) this should generate an error: we're adding an extended property to a CONSTRAINT, but setting @level2name to INDEX. What it actually does is create the extended property as if there really were such a thing as a "primary key index". Interestingly, but perhaps not surprisingly, this new extended property doesn't show up (at least not anywhere I can find) in SSMS. So how do we know it's there? We use the system views, of course!

-- Emit code to drop all extended properties on "primary key indexes". 

SELECT ep.major_id
     , ep.minor_id
     , s.name           AS 'Schema Name'
     , t.name           AS 'Table Name'
     , i.name           AS 'Index Name'
     , ep.name          AS 'EP Name'
     , ep.value         AS 'EP Value'
       
     , 'USE ' + DB_NAME() + '   ' +
       'EXEC sys.sp_dropextendedproperty '
            + '@name = '                                 +  '''' +  ep.name  + ''', '
            + '@level0type = ''SCHEMA'', @level0name = ' + quotename(s.name) + ', ' 
            + '@level1type = ''TABLE'',  @level1name = ' + quotename(t.name) + ', '
            + '@level2type = ''INDEX'',  @level2name = ' + quotename(i.name)   AS 'T-SQL'

  FROM sys.extended_properties    ep
  JOIN sys.indexes                i
    ON ep.major_id = i.object_id
   AND ep.minor_id = i.index_id
  JOIN sys.tables                 t
    ON i.object_id = t.object_id
  JOIN sys.schemas                s
    ON t.schema_id = s.schema_id
 WHERE ep.class_desc    = 'INDEX'
   AND i.is_primary_key = 1
 ORDER BY s.name, t.name, i.name, ep.name

The above displays the offending extended properties on the current database; the last column contains the code necessary to drop each one; for example:

EXEC sys.sp_dropextendedproperty 
    @name       = 'Secret extended property for PK_MyTable index', 
    @level0type = 'SCHEMA', 
    @level0name = [dbo], 
    @level1type = 'TABLE',  
    @level1name = [MyTable], 
    @level2type = 'INDEX',  
    @level2name = [PK_MyTable]
 

Additional Reading

• Modifying and deleting extended properties (sqlservercentral.com)
• Extracting Extended Properties (sqlservercentral.com)
• Industrial-strength database documentation using Extended Properties (sqlservercentral.com)

Adding a Clustered Index - Carefully

This script is useful if you need to add a Primary Key / clustered index on a table, but cannot know if the table already has a clustered index on another column, or if the column you're trying to cluster already has a non-clustered index on it.

-------------------------------------------------------------------------------
-- Constraint_Add_PK_Clustered.sql - Very careful way to add a Primary Key     
--                 backed by a clustered index.  Checks for an existing        
--                 clustered index that was created by a PRIMARY KEY or UNIQUE 
--                 constraint. Checks whether the column is already in use in  
--                 an existing index.                                          
--                                                                             
-- Note: In SSMS, press Ctrl-Shift-M to pop a dialog box for entering values.  
-------------------------------------------------------------------------------
-- Copyright 2010  Larry Leonard, Definitive Solutions Inc.                    
--                 http://www.DefinitiveSolutions.com                          
--                                                                             
-- Copying and distribution of this file, with or without modification, are    
-- permitted in any medium without royalty provided the copyright notice and   
-- this notice are preserved. This file is offered as-is, without any warranty.
-------------------------------------------------------------------------------

DECLARE @sIndexNameToDrop  sysname
DECLARE @sSql              NVARCHAR(4000)
DECLARE @sMsg              NVARCHAR(440)

-- Check for any existing clustered index that was created by a PRIMARY KEY
-- or UNIQUE constraint.  Must use ALTER TABLE to remove these - can't use
-- DROP INDEX on constraints.

SET @sIndexNameToDrop = NULL

SELECT @sIndexNameToDrop = name
  FROM sys.indexes
 WHERE object_id = OBJECT_ID('<schema_name, sysname, dbo>.<table_name_prefix, sysname, tbl><table_name, sysname, ThisTable>')
   AND type_desc = 'CLUSTERED'
   AND (is_unique_constraint = 1 OR is_primary_key = 1)

IF @sIndexNameToDrop IS NOT NULL
BEGIN
    RAISERROR('Dropping existing PRIMARY KEY or UNIQUE constraint %s on table <table_name_prefix, sysname, tbl><table_name, sysname, ThisTable>', 10, 1, @sIndexNameToDrop ) WITH NOWAIT, LOG

    SET @sSql = 'ALTER TABLE <schema_name, sysname, dbo>.<table_name_prefix, sysname, tbl><table_name, sysname, ThisTable> DROP CONSTRAINT ' + @sIndexNameToDrop
    PRINT @sSql
    EXEC(@sSql)
END

-- If there's a non-clustered index, or a clustered index that was created by 
-- an ADD INDEX, and the column we are wanting to cluster is the *only* column 
-- in the index, we want to drop it - *unless* it's a clustered one, and is 
-- *only* on the column we are wanting to cluster.  Since we're only looking 
-- at indexes with only one column, we don't have to check that the column is 
-- not 'included'.

SET @sIndexNameToDrop = NULL

SELECT @sIndexNameToDrop = si.name
  FROM sys.indexes                    AS si
  JOIN sys.index_columns              AS sic
    ON si.object_id  = sic.object_id
   AND si.index_id   = sic.index_id
  JOIN sys.columns                    AS sc
    ON sic.column_id = sc.column_id
   AND sic.object_id = sc.object_id
 WHERE si.object_id  = OBJECT_ID('<schema_name, sysname, dbo>.<table_name_prefix, sysname, tbl><table_name, sysname, ThisTable>')
   AND NOT (sc.name IN ('<column_name, sysname, Column>') AND si.type_desc = 'CLUSTERED')
   AND (si.is_unique_constraint = 0 AND si.is_primary_key = 0)
   AND 1 =
          (SELECT COUNT(*)
             FROM sys.index_columns
            WHERE object_id    = OBJECT_ID('<schema_name, sysname, dbo>.<table_name_prefix, sysname, tbl><table_name, sysname, ThisTable>')
              AND key_ordinal <> 0
              AND column_id   <> 0)

IF @sIndexNameToDrop IS NOT NULL
BEGIN
    RAISERROR('Dropping existing index on table <schema_name, sysname, dbo>.<table_name_prefix, sysname, tbl><table_name, sysname, ThisTable>, column <column_name, sysname, Column>', 10, 1) WITH NOWAIT, LOG

    SET @sSql = 'DROP INDEX ' + @sIndexNameToDrop + ' ON <schema_name, sysname, dbo>.<table_name_prefix, sysname, tbl><table_name, sysname, ThisTable>'
    PRINT @sSql
    EXEC(@sSql)
END

-- Now add the clustered index.
RAISERROR('Adding <set_as_primary_key, NVARCHAR, PRIMARY KEY> clustered index to <database_name, sysname, MyDB>''s <table_name_prefix, sysname, tbl><table_name, sysname, ThisTable> table', 10, 1) WITH NOWAIT, LOG

ALTER TABLE <schema_name, sysname, dbo>.<table_name_prefix, sysname, tbl><table_name, sysname, ThisTable> WITH CHECK
  ADD CONSTRAINT PK_<table_name, sysname, ThisTable> <set_as_primary_key, NVARCHAR, PRIMARY KEY> CLUSTERED (<column_name, sysname, Column>)
 WITH (PAD_INDEX = ON, FILLFACTOR = <fill_factor, INT, 70>)

See BOL for info on how to create and use templates.