Linked Servers. Functionality and Performance Considerations for Linked Servers

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1 Linked Servers Linked servers provide SQL Server with access to data from remote data sources. Depending on the remote data source, you can issue queries, perform data modifications, and execute remote procedure calls. Remote data sources can be homogeneous (other SQL Server instances) or heterogeneous (other relational database products and data sources such as DB2, Access, Oracle, Excel, and text files). Cross-platform queries allow you to access legacy database systems, without the cost of merging or migrating existing data sources. SQL Server connects to the remote data sources with an OLE DB provider. OLE DB, created by Microsoft, is a set of COM (component object model) interfaces used to access tabular data from varying data sources. An OLE DB provider is used to define a linked server on the local server. OLE DB providers are installed automatically with MDAC. The design of the OLE DB provider determines what kind of operations can be implemented through a distributed query (SELECT, INSERT, UPDATE, DELETE, stored procedure execution). You can also run distributed queries without having to define linked servers, by using the Transact-SQL functions OPENROWSET and OPENDATASOURCE. Like linked servers, OPENROWSET and OPENDATASOURCE use OLE DB to connect to the remote data source; however, these functions do not encompass all the features included with linked servers, and should only be used for temporary or once-only queries. When a Transact-SQL statement uses a linked server, the OLE DB provider for the linked server is loaded into the SQL Server memory address space, if it is not already loaded. SQL Server takes the Transact-SQL statement and calls the OLE DB interface. The OLE DB interface is then responsible for contacting the remote data source. The OLE DB provider processes the result set, and the output is then returned to the SQL Server client that made the initial call. Checklist Functionality and Performance Considerations for Linked Servers If you have another way to exchange or extract data, consider it. Linked servers require careful planning and have some performance drawbacks if used for heavy transaction processing or complicated decision-support reporting.

2 Chapter 9 Do not use linked servers instead of SQL Server replication. If you have two SQL Server instances that need to exchange data, consider replication over linked servers. Watch out for very large data sets and complicated queries. Do not assume that queries that run quickly on your local server will run as well in a distributed query to a linked server. Join operations, aggregate functions, ORDER BY clauses, and grouping can have harmful effects on your query response time. Read the fine print on your OLE DB provider. If the OLE DB provider for your database product is not supported, believe it. The fact that you can get one or two queries to work doesn't guarantee stability or extensibility down the road. Not all OLE DB providers, supported or otherwise, provide the same functionality. Understand what you can and cannot do with your heterogeneous OLE DB provider. Distributed queries require ANSI_NULLS and ANSI_WARNINGS connection settings to be set ON. Keep in mind that enabling these settings within a stored procedure will cause SQL Server to recompile the stored procedure, which may decrease some of the performance benefits of using a stored procedure. To use a specific OLE DB provider, you need to install the OLE DB provider drivers on your SQL Server computer. SQL Server will attempt to process certain relational database operations on the remote server side. This does not mean that it will use the most efficient method, or that certain operations (joins, aggregate functions, sorting, grouping) will always run on the remote server. Monitor very closely what behavior your distributed queries use, by checking the execution plan when performing initial testing. You cannot use query hints within distributed queries (see Chapter 15, for more details on query hints). If you do, you will get the error message "7277", "Cannot specify an index or locking hint for a remote data source". 9.1 How to... Add a Linked Server To add a linked server using Enterprise Manager: Expand the SQL Server Group and registration. Expand the Security folder. Right-click Linked Server and select New Linked Server. Enter the remote server name in the Linked Server text box. For SQL Server instances, TCP/IP addresses can be used if TCP/IP is an enabled protocol for the remote data source and SQL Server instance. Select the Server type, either SQL Server or Other data source OLE DB provider. If you select SQL Server, the other text boxes will be grayed out. If you choose a different data source provider, the list of drop-down selections will be based on the drivers included with SQL Server, and any other OLE DB drivers on your server (from separately installed packages). SQL Server includes OLE DB providers for Microsoft Jet 4.0, DTS packages, Indexing Service, Internet Publishing, Microsoft Search, ODBC Drivers, OLAP Services 8.0, Oracle, Microsoft Directory Services, Simple Provider, and, of course, SQL Server. The OLE DB provider you select determines the values you choose for Product name, Data source, Provider string, Location, and Catalog: 522

3 Linked Servers The Provider name shows the name of the OLE DB provider used to connect to the remote server. The Data source specifies the data source name property, which can be the server name, DSN name, or system path for the database. The Provider string may vary, based on the OLE DB provider used, and sets a unique data source string required by some OLE DB types. Location specifies the OLE DB provider location of the database, and is not required by all providers. The Catalog field sets the default database for the linked server connection. For a better understanding of what values to choose for different OLE DB providers, see the sp_addlinkedsrvlogin OLE DB provider options later in this section. Selecting the Provider Options button will provide further configuration options: Dynamic parameters Nested queries Level zero only Allow InProcess If enabled, the parameter marker (?) sign is allowed. If enabled, the OLE DB provider allows nested SELECT statements in the FROM clause. If enabled, only level 0 OLE DB interfaces are invoked against the provider. This means that the provider is responsible for executing, optimizing, and generating the rowset. The SQL Server instance making the linked server call is only responsible for making the initial call and receiving the rowset. If enabled, the provider runs in process to the SQL Server process. This means that the OLE DB provider runs within the memory address space of SQL Server. This allows more functionality than out-of-process, but places the SQL Server process at risk if provider errors occur. Table continued on following page 523

4 Chapter 9 Non transacted updates Index as access path Disallow ad hoc accesses If enabled, provider updates are allowed, but are not supported as a transaction (and, therefore, SQL Server can't roll back or recover any data modifications if an error occurs). For example, you cannot use the BEGIN TRAN, COMMIT, or ROLLBACK commands. You would not be able to roll back a distributed update to a table, and partial updates would not necessarily be recovered. (For more information on transactions, see Chapter 5). If enabled, SQL Server will utilize execution methods that use indexes on the data source. This may benefit performance of the distributed query (depending on the indexes on the data source, and the query construction). If enabled, no ad hoc access is allowed via the OPENROWSET or OPENDATASOURCE Transact-SQL functions. This will help you limit queries, by forcing them to use a defined linked server only. In SQL Server 2000 Service Pack 3, ad hoc access to OLE DB providers will not be allowed by default. If you want ad hoc access you must explicitly set this option to zero. Starting with SQL Server 2000 Service Pack 3, a new provider option called SqlServerLike has been introduced. When this option is non-zero, SQL Server can push queries that use the LIKE predicate to the provider for processing. In previous versions, if the provider was not a SQL Server OLE DB provider, any distributed queries using LIKE were evaluated and processed locally on the calling SQL Server instance. This option can potentially improve performance for queries that use the LIKE predicate. You cannot configure the SqlServerLike option using Enterprise Manager. Instead, it must be enabled via a registry key modification. Use a registry editor to locate the registry key HKEY_LOCAL_MACHINE\Software\Microsoft\MSSQLServer\Providers \<ProviderName> for default instances, or HKEY_LOCAL_MACHINE\Software\ Microsoft\Microsoft SQL Server\<Instance Name>\Providers\ <ProviderName> for named instances. Under the specified provider <ProviderName> key, add a DWORD value, and name it SQLServerLike. Then set the value to 1 to enable, or 0 to disable it. In the Security tab, you can specify which login will be used for data access to the provider. In the Local server login to remote server login mappings: section, you can map one or more local logins to associated remote users. Specify the Local Login field for the local login to map. Select the Impersonate checkbox if the local ID matches the remote server ID and has the proper permissions. Select Remote User and Remote Password for the remote login and password to which to map the local login. Note that the remote login and password can't be a WinNT username and password. You can configure security for those connecting logins not found in the top section by setting the options in the For a login not defined in the list above, connections will: section. To restrict access only to those in the local server login list, select Not be made. For data sources that do not have a security context (for example, text files), select Be made without using a security context. If you wish the non-listed local login to impersonate a remote login (map to the same name and password on the remote server side), select Be made using the login's current security context. Select Be made using this security context: to select a remote login and password that will be used to connect to the remote data source for all unlisted local logins. When selecting logins for use with linked servers, choose a remote login with the lowest level of permissions that are needed to do the job. 524

5 Linked Servers The Server Options tab allows you to set additional linked-server options: Collation compatible Data access RPC RPC out Use remote collation Collation name Collation Compatible is used for SQL Server-to-SQL Server communication. Enable this setting if you are certain the local SQL Server instance has the same collation as the remote SQL Server instance. Doing so will improve performance, as SQL Server will no longer have to perform comparisons of character columns between the data sources, as the same collation is assumed. If enabled, distributed query access is allowed. Enables remote procedure calls from the server. Enables remote procedure calls to the server. Determines if remote server collation is used instead of the local server collation. If Use Remote Collation is enabled, you need to specify the name of the remote server collation. This name must be supported by SQL Server, and is used to associate a common and recognized SQL Server collation to the remote server. Table continued on following page 525

6 Chapter 9 Connection timeout Query timeout Specifies the number of seconds a connection attempt will be made to the linked server before a timeout occurs. Determines the number of seconds after which a waiting query will time out. If this value is 0, then the sp_configure value configured for the query wait option will be used instead (see Chapter 2 for more details on this option). Select OK to create the connection. Test the new linked server by expanding the server name node and clicking Tables or Views. Check the right-hand pane of Enterprise Manager to see what objects are listed (this will show default database objects based on the login). To remove a linked server entry, right-click the linked server, and select Delete. You will be prompted to check if you are sure about removing the linked server. If you have local logins associated with remote logins, you will receive an additional prompt: 526

7 Linked Servers To add and configure a linked server using Transact-SQL, use the following stored procedures: Stored Procedure sp_addlinkedserver sp_addlinkedsrvlogin sp_droplinkedsrvlogin sp_serveroption sp_dropserver Creates a linked server. Maps a local login to a remote login. Drops a login mapping for a linked server. Sets server options for a linked server. Drops a linked server definition. To add a new linked server, execute sp_addlinkedserver. The syntax from SQL Server Books Online is as follows: sp_addlinkedserver = ] 'server' [, = ] 'product_name' ] [, = ] 'provider_name' ] [, = ] 'data_source' ] [, = ] 'location' ] [, = ] 'provider_string' ] [, = ] 'catalog' ] The following are parameter descriptions; keep in mind that not all fields are required for each OLE DB provider type, and that they also vary according to the product as well. At the end of this section, we will review different connection types (including DB2, Jet, and Oracle): = ] 'server' Local name used for linked server. Instance names are allowed, for example DOPEY\DOPEY. = ] 'product_name' ] Product name of OLE DB data source. = ] 'provider_name' ] Unique programmatic identifier for the OLE DB provider. = ] 'data_source' ] Data source as interpreted by the specified OLE DB provider. = ] 'location' ] Location as interpreted by the specified OLE DB provider. = ] 'provider_string' ] Connection string specific to the OLE DB provider. = ] 'catalog' ] catalog sometimes refers to database, but it can also be defined by OLE DB provider interpretation. Use the xp_enum_oledb_providers extended stored procedure to list all available OLE DB providers on your database server. This prevents you adding a linked server that references an OLE DB provider that is not on the database server. This is an undocumented stored procedure, so it should not be used within application code or procedures. Only use this procedure as a helpful ad hoc tool. To use the procedure, execute the following command in Query Analyzer: 527

8 Chapter 9 Example 9.1.1: Adding a linked server Adding a linked server referencing a SQL Server instance: USE master GO EXEC sp_addlinkedserver 'BOOKREPOSITORY', N'SQL Server' GO Example 9.1.2: Adding a linked server referencing a named instance Adding a linked server referencing a SQL Server named instance: USE master GO EXEC sp_addlinkedserver 'BOOKREPOSITORY\BOOKDB1', N'SQL Server' GO OLE DB Provider s Other OLE DB providers require different parameters to create a linked server. The following section reviews required sp_addlinkedserver parameters for DB2, Jet, Oracle, ODBC valid system DSNs, and FoxPro. DB2 Install Windows NT Client for Host Integration Server 2000 on the SQL Server computer. Install the OLE DB Provider for DB2 and the proper network components for an SNA network. 528

9 Linked Servers You can also connect to DB2 via OLE DB for ODBC, by creating a system DSN with the DB2 Connect utility. Create the data source after installing DB2 Connect: server product_name provider_name provider_string catalog Value Server name Microsoft OLE DB DB2OLEDB Provider string example: 'NetLib=SNA; NetAddr=; NetPort=; RemoteLU=XXXXXXX; LocalLU=LOCAL; ModeName=XXXXXX; User ID=XXXXXX; Password=XXXXXX; InitCat=XXXXXXX; DefaultSchema=XXXXXX; PkgCol=XXXXXX; TPName=; Commit=YES; IsoLvl=NC; AccMode=; CCSID=37; PCCodePage=1252; BinAsChar=NO; Data Source=XXXXXXX' DB2 catalog Jet In addition to Microsoft Access, the Jet provider can also be used to connect to an Excel spreadsheet or text file: server product_name provider_name data_source Value Server name Access Microsoft.Jet.OLEDB.4.0 C:\path\databasename.mdb 529

10 Chapter 9 OLE DB for ODBC OLE DB for ODBC allows you to connect to most valid ODBC data sources configured on your server. If you do not have an OLE DB provider specific to the vendor product, try OLE DB for ODBC. Before adding a linked server, create a system data source on the SQL Server computer. To add a data source, select the Start menu, Settings Control Panel Administrative Tools. Double-click Data Sources (ODBC) for a Windows NT 4.0 or Windows 2000 server. Select User DSN, System DSN, or File DSN tab, and select the Add button. Specify the type of data source you wish to set up, and any other user-defined options that may be required for that provider type: server provider_name data_source Value Server name MSDASQL System data source name If you do not want to create a system data source on the SQL Server computer, you can create a DSN-less connection. For example: server product_name provider_name data_source location provider_string Value Server name NULL MSDASQL NULL NULL A valid provider string. For SQL Server for example: 'Driver={SQL Server}; Database=databasename; Server=ServerName; UID=loginid;PWD=pwd;' OLE DB for SQL Server 7.0 and 2000 Use OLE DB for SQL Server for versions SQL Server 7.0 and You can use OLE DB for ODBC, but it does not perform as well: server provider_name data_source Value Linked server name SQLOLEDB Network server name 530

11 Linked Servers OLE DB for Oracle Install Oracle client software on SQL Server computer. Make sure to reboot the server after installation: server product_name provider_name data_source Value Server name Oracle MSDAORA SQL*Net alias name FoxPro FoxPro connections can be made via OLE DB for ODBC too. The following example details a connection with a DSN: server provider_name provider_string Value Server name MSDASQL 'Driver={Microsoft Visual FoxPro Driver}; UID=; PWD=; SourceDB=<directory>\databasename.dbc; SourceType=DBC; Exclusive=No; BackgroundFetch=Yes; Collate=Machine;' Sybase With Sybase being so much closer in the database family tree to SQL Server than, say, DB2, you would think that connectivity would be documented and supported via linked servers. Sybase does provide an OLE DB provider beginning with version 12, and can also be connected to via OLE DB for ODBC. At the time of writing, an official SQL Server 2000 provider test has not been blessed by either Sybase or Microsoft, so until then, you should think twice about staking your application on under-documented and ambiguously supported technologies. Active Directory You can connect to Active Directory using the Microsoft OLE DB Provider for Microsoft Directory Services. Use OPENQUERY to send a command to the Active Directory Service. Use the values provided below for setting up the linked server Active Directory data source: server Value User-defined name of the linked server product_name Active Directory Services 2.5 provider_name data_source ADsDSOObject adsdatasource 531

12 Chapter 9 Adding Login Mappings to a Linked Server Once you have added your linked server with sp_addlinkedserver, you can add login mappings from your local SQL Server instance to the linked server OLE DB provider. The syntax for sp_addlinkedsrvlogin from SQL Server Books Online is as follows: sp_addlinkedsrvlogin = ] 'rmtsrvname' [, = ] 'useself' ] [, = ] 'locallogin' ] [, = ] 'rmtuser' ] [, = ] 'rmtpassword' ] = ] Name of the linked server for the login mapping. = ] If true (true is the default), SQL Server-authenticated logins use the same login credentials at the remote server. This will not work if account delegation and Windows authentication isn't supported on the provider server. Account delegation requires that all the servers that you connect to are running Windows 2000, use Microsoft Active Directory, and have Kerberos support enabled. Kerberos is a means of supporting mutual authentication and the ability to pass security credentials across servers and clients. = ] Local login to map from the local SQL Server instance. = ] User name on remote server to map to. = ] Password on remote server for a remote user. Example 9.1.3: Allowing connections using local logins EXEC = = 'true' Example 9.1.4: Mapping a domain login to a local user EXEC = @rmtpassword='bookfun' A tip regarding sp_addlinkedsrvlogin: do not forget to include = 'false' for login mappings, otherwise your remote user and password parameters will be ignored, and SQL Server will attempt to use your local login on the remote server. Setting Server Options To configure the options of a linked server, use the sp_serveroption procedure. The syntax from SQL Server Books Online is as follows: 532

13 Linked Servers sp_serveroption =] 'server', =] 'option_name', =] 'option_value' This procedure takes the linked server name as the first parameter, and then a name and value pair for the configurable properties, which are as follows: Property Collation compatible Collation name Connect timeout Data access Lazy schema validation Query timeout RPC RPC out Use remote collation For SQL Server-to-SQL Server communication. Enable this setting if you are certain the local SQL Server instance has the same collation as the remote SQL Server instance. If Use Remote Collation is enabled, specifies the name of the remote server collation. This name must be supported by SQL Server, and is used to associate a common and recognized SQL Server collation to the remote server. Sets the number of seconds a connection attempt will be made to the linked server before a timeout occurs. If enabled, distributed query access is allowed. If set to true, the schema is not checked on remote tables at the beginning of the query. Although this reduces overhead for the remote query, if the schema has changed on the linked server data source after the query is compiled but before it is executed, an error could occur. The number of seconds after which a waiting query will time out. Enables remote procedure calls from the server. Enables remote procedure calls to the linked server data source. Determines whether the remote server collation is used instead of the local server collation. When this option is enabled, the remote data source column collation is used (for SQL Server linked servers), and the collation specified in the Collation Name option is then used for heterogeneous data sources. Example 9.1.5: Designating that the collation is compatible EXEC sp_serveroption 'BOOKREPOSITORY', 'collation compatible', 'true' Example 9.1.6: Enabling remote procedure calls to and from the server EXEC sp_serveroption 'BOOKREPOSITORY', 'rpc', 'true' EXEC sp_serveroption 'BOOKREPOSITORY', 'rpc out', 'true' Example 9.1.7: Disabling data access EXEC sp_serveroption 'BOOKREPOSITORY', 'data access', 'false' 533

14 Chapter 9 Example 9.1.8: Dropping a linked server login To drop a linked server entry from your local server, you must first drop associated login mappings. One method for dropping login mappings is to use sp_droplinkedsrvlogin. This stored procedure takes the linked server name for the first parameter, and the local login for the second parameter: EXEC sp_droplinkedsrvlogin 'BOOKREPOSITORY', 'sa' Example 9.1.9: Dropping a linked server entry and associated logins After you have finished dropping all login mappings, you can drop the linked server entry with the sp_dropserver stored procedure. This procedure takes two parameters: the first parameter uses the linked server name to drop and the second accepts either NULL or droplogins. This is an alternative method to dropping all logins associated with a linked server entry, if you didn't already do so with sp_droplinkedsrvlogin: EXEC sp_dropserver 'BOOKREPOSITORY', 'droplogins' 9.2 How to... Execute Distributed Queries with a Linked Server Distributed queries can reference one or more linked servers, performing either read or update operations against the individual linked servers. In general, Microsoft SQL Server requires distributed transactions support from the corresponding OLE DB provider whenever data from more than one linked server is likely to be updated in a transaction. The types of queries that are supported against linked servers depend on the level of support for transactions present in the OLE DB providers. You can run a distributed query referencing a linked server by using either a four-part name or the OPENQUERY Transact-SQL command. The basic syntax for referencing a linked server using a four-part name is as follows: linkedservername.databasename.owner.object Example 9.2.1: Querying a linked server using the four-part name 534

15 Linked Servers You can also query a linked server using the OPENQUERY function. The syntax is: OPENQUERY (linked_server_name, 'query') OPENQUERY acts like a table rowset, taking the linked server name as the first parameter and a valid query as the second parameter. No variables are allowed in the second parameter, and the types of queries allowed (SELECT, INSERT, UPDATE, and DELETE) depend on the OLE DB provider that is used. Note also that, although the query can return multiple result sets, only the first result set is displayed. Example 9.2.2: Querying a named instance linked server To query a linked server that is also a named instance of SQL Server, you must put the server's name in square brackets: SELECT * FROM [BOOKREPOSITORY\BOOKDB1].BookRepository09.dbo.Authors Example 9.2.3: Executing a stored procedure on the remote data source The following example executes the sp_lock procedure on a linked server named BOOKREPOSITORY. The linked server must have RPC Out enabled, and the OLE DB provider must allow remote procedure calls. The security of the remote login used must also have the appropriate permissions: EXEC BOOKREPOSITORY.master.dbo.sp_lock Example 9.2.4: Modifying data on the remote data source The following examples show how to INSERT, UPDATE, and DELETE data on the remote data source. The OLE DB provider must allow data modifications. The security of the remote login used must also have the appropriate permissions: INSERT BOOKREPOSITORY.BookRepository09.dbo.Authors (vchlastname, vchfirstname) VALUES ('Sack', 'Joe') UPDATE BOOKREPOSITORY.BookRepository09.dbo.Authors SET vchfirstname = 'Joseph' WHERE vchlastname = 'Sack' AND vchfirstname = 'Joe' DELETE BOOKREPOSITORY.BookRepository09.dbo.Authors WHERE vchlastname = 'Sack' AND vchfirstname = 'Joseph' Example 9.2.5: Querying a linked server with OPENQUERY 535

16 Chapter 9 Remember that linked query behavior is not always predictable. SQL Server will attempt to have certain operations processed remotely but, depending on the operation and meta data available from the OLE DB provider, the best query plan may not always be selected. Example 9.2.6: Viewing a query plan for a distributed query You can view the query execution plan to see where processes will take place. For example, the following query joins data from a remote table and a local table: SELECT LocalPubs.* FROM OPENQUERY (BOOKREPOSITORY, 'SELECT * FROM BookRepository09.dbo.Authors WHERE vchfirstname = ''Johnson''') AS RemotePubs, BookRepository09.dbo.Authors AS LocalPubs WHERE RemotePubs.iAuthorID = LocalPubs.iAuthorID This query uses the RemotePubs alias for the OPENQUERY result set and the LocalPubs alias for the local table join (you can also join remote tables or views to local tables and views). To view the query plan in Query Analyzer, go to the Query menu and select Show Execution Plan (you can also choose Show Estimated Execution Plan if you do not wish to run the actual query). Execute the query and look at the Execution Plan tab: This execution plan tells us that 97% of the processing took place in a remote scan operation on the linked server. See Chapter 17 for more information on the graphical query plan tool. 9.3 How to... Execute Ad Hoc Distributed Queries using OPENROWSET and OPENDATASOURCE If you only require a temporary method of accessing a remote server, SQL Server provides the Transact-SQL commands OPENROWSET and OPENDATASOURCE. These commands should be used as temporary or one-time use methods, as defined linked servers generally perform better. OPENROWSET acts like a table in a SELECT statement, except that it takes OLE DB data source information and uses it to access a remote server. The syntax from SQL Server Books Online is as follows: OPENROWSET ( 'provider_name', { 'datasource' ; 'user_id' ; 'password' 'provider_string' }, { [ catalog. ] [ schema. ] object 'query' } ) 536

17 Linked Servers provider_name datasource user_id password provider_string { [ catalog. ] [ schema. ] object query } Name describing the OLE DB provider. OLE DB data source. User ID on remote data source. Password on remote data source. If you do not specify the first four parameters, you can use an OLE DB provider string instead. Choose a specific catalog (database), schema (object owner), and object, or a specific query. The query is processed at the provider. OPENROWSET accepts the same OLE DB provider parameters as sp_addlinkedserver, so refer to the earlier OLE DB Provider s section for more details. Warning: Using rowset functions like OPENDATASOURCE in production code can be a security risk if you specify the password of a remote login in the call. The OPENDATASOURCE function, like OPENROWSET, also bypasses the linked server functionality. This function acts as a result set table and can be used with a SELECT query. The difference between OPENDATASOURCE and OPENROWSET, however, is that OPENDATASOURCE acts as the first part of a four-part name whereas OPENROWSET sits in place of the entire four-part name. The syntax from SQL Server Books Online is as follows: OPENDATASOURCE ( provider_name, init_string ) provider_name init_string Name of the registered ID for the OLE DB provider. Specifies a connection string, using keyword-value pairs. Example 9.3.1: Running a server-side query using OPENROWSET In this example, we will run a server-side query using OPENROWSET, and return the top 10 rows: SELECT TOP 10 * FROM OPENROWSET ('SQLOLEDB', 'BOOKREPOSITORY';'sa';'xxxx', 'SELECT * FROM BookRepository09.dbo.Authors') Example 9.3.2: Using Windows authentication with OPENROWSET You can connect to other SQL Server instances using Windows authentication. To do so, use a name/value property setting of Trusted_Connection=Yes. For example: 537

18 Chapter 9 SELECT TOP 10 * FROM OPENROWSET ('SQLOLEDB', 'SERVER=BOOKREPOSITORY;Trusted_Connection=yes', 'SELECT * FROM BookRepository09.dbo.Authors') Example 9.3.3: Selecting data from a table using OPENDATASOURCE SELECT TOP 10 * FROM OPENDATASOURCE ('SQLOLEDB', 'Data Source=BOOKREPOSITORY;User ID=sa;Password=xxxx' ).BookRepository09.dbo.Authors Example 9.3.4: Using Windows authentication with OPENDATASOURCE You can connect to other SQL Server instances using Windows authentication. To do so, use a name/value property setting of Integrated Security=SSPI. For example: SELECT * FROM OPENDATASOURCE ('SQLOLEDB', 'Data Source=BOOKREPOSITORY;Integrated Security=SSPI' ).BookRepository09.dbo.Authors 9.4 How to... List All Tables from a Linked Server using sp_tables_ex To return information about a table on the linked server, run the procedure sp_tables_ex. The syntax from SQL Server Books Online is as follows: sp_tables_ex = ] 'table_server' [, = ] 'table_name' ] [, = ] 'table_schema' ] [, = ] 'table_catalog' ] [, = ] 'table_type' ] = ] Name of linked server = ] Name of table on linked server = ] Name of schema (usually object owner) = ] Name of catalog (usually database) 538

19 Linked Servers = ] Type of table (default NULL), allowing values: ALIAS GLOBAL TEMPORARY (global temp table) LOCAL TEMPORARY (local temp table) SYNONYM SYSTEM TABLE TABLE (user table) VIEW Example 9.4.1: Listing all tables for a specific database on a linked server 9.5 How to... Show Privilege Information about a Linked Server Table with sp_table_privileges_ex To display table permissions for a linked server table, use the sp_table_privileges_ex stored procedure. The syntax from SQL Server Books Online is as follows: sp_table_privileges_ex = ] 'table_server' [, = ] 'table_name' ] [, = ] 'table_schema' ] [, = ] 'table_catalog' ] 539

20 Chapter 9 = ] Linked server name = ] Specific table name = ] Table schema (usually object owner) = ] Table catalog (usually database) Example 9.5.1: Listing privileges for a specific table on a linked server 9.6 How to... Display Column Data for a Linked Server Table using sp_columns_ex To return column information about a specific linked server table or tables, use the stored procedure sp_columns_ex. The syntax from SQL Server Books Online is as follows: sp_columns_ex = ] 'table_server' [, = ] 'table_name' ] [, = ] 'table_schema' ] [, = ] 'table_catalog' ] [, = ] 'column' ] [, = ] 'ODBCVer' ] = ] Linked server name = ] Table name = ] Schema name for table (usually relates to owner) = ] Catalog name for table (sometimes relates to database) 540

21 Linked Servers = ] Column name = ] Version of ODBC being used, either 2 or 3 Example 9.6.1: Listing all columns for a specific table Example 9.6.2: Listing data for one column 9.7 How to Return Column Privileges for a Linked Server Table using sp_column_privileges_ex To list privileges for columns on a specific table for a linked server, use sp_column_privileges_ex. The syntax from SQL Server Books Online is as follows: sp_column_privileges_ex = ] 'table_server' [, = ] 'table_name' ] 541

22 Chapter 9 [, = ] 'table_schema' ] [, = ] 'table_catalog' ] [, = ] 'column_name' ] = ] Linked server name = ] Table name = ] Schema name (usually object owner) = ] Catalog name (usually database) = ] Column name Example 9.7.1: Return permissions for a specific column 9.8 How to... List Linked Servers Defined on Your Local Server To list all defined linked servers on your local server, execute sp_linkedservers. The system stored procedure returns the following information: Column SRV_NAME SRV_PROVIDERNAME SRV_PRODUCT Linked server name OLE DB provider name Database product name 542

23 Linked Servers Column SRV_DATASOURCE SRV_PROVIDERSTRING SRV_LOCATION SRV_CAT OLE DB data source name OLE DB provider string OLE DB location OLE DB catalog 9.9 How to Troubleshoot Linked Servers and Distributed Queries As with many SQL Server issues and bugs, making use of Internet references will ensure you have the latest information. The chances are that if you encounter an error, someone else has encountered that error too. Make use of Microsoft's Knowledge Base site along with message boards and news groups. The following are common errors in distributed queries for linked servers. Keep in mind that the error number is usually listed as Msg instead of error in the error text. For example, Query Analyzer will display error 7399 as Server: Msg 7399 : Beginning with SQL Server 2000 Service Pack 3, error messages returned from a distributed query will return more detailed error information if the OLE DB Provider of the linked server supports the IErrorRecords OLE DB interface. Error Number Message 115 '<Linked-Server Name>' is not a recognized OPTIMIZER LOCK HINTS option. You may receive this error if running an OPENQUERY function within the context of a database with compatibility level 65 or lower (SQL Server 6.5). Change your local database compatibility to 70 (7.0) or greater. Table continued on following page 543

24 Chapter 9 Error Number Message 7303 Could not initialize data source object of OLE DB provider '%ls'. %ls 7306 Could not open table '%ls' from OLE DB provider '%ls'. %ls 7314 OLE DB provider '%ls' does not contain table '%ls'. The table either does not exist or the current user does not have permissions on that table An error occurred while preparing a query for execution against OLE DB provider '%ls'. %ls The linked server setup parameters may be incorrect or the login is invalid. Implies that the OLE DB provider doesn't support data modification statements. Indicates the table does not exist, user permissions are insufficient, or the server is case-sensitive and another case should be used for the object name. This error suggests a query string syntax error. Make sure you are using the proper syntax for the OLE DB provider being accessed. This error also occurs if you are using a four-part name, with an OLE DB connection to DB2 on an AS/400 platform. If the OLE DB property Nested Queries is enabled, you will receive error To resolve this problem, you can either disable Nested Queries or use OPENQUERY instead of a four-part name. See Microsoft Knowledge Base article Q PRB: Error 7321 Occurs When You Run a Query That Uses Four-Part Names Against DB2 on an AS/400 Computer OLE DB provider '%ls' supplied inconsistent metadata for a column. Metadata information was changed at execution time. This occurs when inconsistent column meta data is returned between compilation and execution time for the query. To troubleshoot this, you need to determine which column or columns are responsible for the inconsistent meta data. For example, you can get this error if connecting to an Oracle table that did not specify nullability of a column. See Microsoft Knowledge Base Article Q PRB: Distributed Queries Return Error 7356 with MSDAORA for more details. 544

25 Linked Servers Error Number Message 7357 Could not process object '%ls'. The OLE DB provider '%ls' indicates that the object has no columns The operation could not be performed because the OLE DB provider '%ls' was unable to begin a distributed transaction Could not start a transaction for OLE DB provider '%ls'. or: Only one transaction can be active on this session OLE DB provider '%ls' reported an error. %ls Cannot start your application. The workgroup information file is missing or opened exclusively by another user. This error suggests that you chose either invalid column names or table names, or that you have insufficient permissions to access the object. Indicates that the OLE DB provider may not support distributed transactions for data modification statements. This error also occurs when trying to connect a distributed transaction through a firewall. MS DTC uses dynamic port allocation to bind a distributed transaction, and the port can be blocked by the firewall. To select one port for use by MS DTC and prevent dynamic port allocation, follow the procedure in Microsoft Knowledge Base article Q INFO: Configuring Microsoft Distributed Transaction Coordinator (DTC) to Work Through a Firewall. Indicates that the OLE DB provider does not support nested transactions. By selecting SET XACT_ABORT ON, SQL Server will not require nested-transaction support from the provider. SQL Server Books Online suggests that this is specifically a Microsoft Access-related error, due either to the Microsoft Access database not being configured with an Admin account without a password, or your registry key not pointing to the correct Access workgroup file. This message can also occur, however, after installing Oracle 8.x client software on the server. The error occurs if you try to connect with the Oracle OLE DB connection without first restarting the server after installing the Oracle 8.x client software. See Microsoft Knowledge Base Article Q PRB: Distributed Query Against Oracle 8.x Fails for more details. Table continued on following page 545

26 Chapter 9 Error Number Message 7403 Could not locate registry entry for OLE DB provider '%ls' Could not perform a Windows NT-authenticated login because delegation is not available Error converting data type %ls to %ls MS DTC on server '%.*ls' is unavailable The server 'Servername' already exists. Slow Response Time Slow response time for a query with multiple OR clauses within a stored procedure or OLE DB for ODBC connection. Either you used an incorrect OLE DB provider name when adding the linked server, or the OLE DB drivers are not registered correctly. The error indicates that mapping has not been performed to a remote login and password. This can occur when security delegation is not supported. This error indicates that the OLE DB provider has returned a datetime value outside the supported range. Omit the column from your query, or add a WHERE clause that eliminates invalid or out-of-range dates. Data modifications require the MSDTC service to be running locally, as well as provider support for distributed transactions. Start the MSDTC service on your local server. This error is received when you attempt to add a new linked server. Server information for both linked servers and replication servers is kept in the sysservers table. If replication configuration has already added an entry for the server you are attempting to add, you may get this error. To resolve this issue, you need only change server status to allow data access. The instructions for doing this are fully described in the Microsoft Knowledge Base article , PRB: Adding a Linked Server Causes Error If you are using multiple OR clauses for fields in a composite index, the execution time may be sub-optimal as an index or table scan may occur instead of using the composite index. This is a known bug and is referenced in Microsoft Knowledge Base Article Q223423, BUG: Optimizer Uses Scan with Multiple OR Clauses on Composite Index. This Knowledge Base article gives several suggestions for workarounds. 546

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