Several maintenance activities should be performed
regularly. Over time, a database becomes fragmented and is likely to
become much larger than needed. You might want to shrink database files
because setting them to automatically shrink causes a performance hit on
the system.
You should
configure maintenance plans for most of the databases on a server to
ensure that they are operating optimally. You can use maintenance plans
to create a workflow of the maintenance tasks required to make sure that
your database performs well, is regularly backed up in case of system
failure, and is checked for inconsistencies.
You can use the
Maintenance Plan Wizard to create core maintenance plans and to initiate
maintenance workflows. After you create plans by using this wizard, you
can make modifications and additions to these plans manually, which
gives you quite a bit of flexibility. In the SQL Server 2005 database
engine, maintenance plans create a job that performs these maintenance
tasks automatically at scheduled intervals.
Using the Database Console Command (DBCC)
A number of DBCC operations should be performed periodically. You can effectively use many of the DBCC options in troubleshooting. The following options can provide valuable data to help you determine how to improve performance:
DBCC SHOW_STATISTICS—Displays the current distribution statistics for the specified target on the specified table.
DBCC SQLPERF—Provides statistics about the use of transaction log space in all databases.
DBCC TRACESTATUS—Displays the status of trace flags.
DBCC USEROPTIONS—Returns the SET options that are active (that is, set) for the current connection.
Alert
DBCC SHOW_STATISTICS displays the current statistic distribution, and DBCC SQLPERF provides information about available log space. All DBCC operations will be supported going forward, unlike the dynamic views sys.dm_db_index_operationalstats and sys.dm_db_index_physicalstats, which provide the same information but may not be supported in the next version.
DBCC Validation Operations
After status information
has been generated, the next task in information retrieval is obtaining
validation data, which can also give you insight into a server. The
validation options represent tools that can reveal database storage
problems and also provide the mechanisms to modify and fine-tune the
environment. To validate various objects on a server, you can use the
following:
DBCC CHECKALLOC—Checks the consistency of disk space.
DBCC CHECKCATALOG—Checks for consistency in system tables.
DBCC CHECKCONSTRAINTS—Checks the integrity of constraints.
DBCC CHECKDB—Checks the allocation and structural integrity of all the objects in a database.
DBCC CHECKFILEGROUP—Checks the allocation and structural integrity of tables in a filegroup.
DBCC CHECKIDENT—Checks the current identity value for a table and, if needed, corrects the value.
DBCC CHECKTABLE—Checks the integrity of the data, index, text, ntext, and image pages.
DBCC NEWALLOC—Checks the allocation of data and index pages. It is equivalent to CHECKALLOC and is used for backward compatibility only.
Note
Use of CHECKALLOC is unnecessary if CHECKDB is used first. CHECKDB contains a superset of options that includes all the functionality provided by CHECKALLOC.
Some of these functions are CPU and disk intensive. You should exercise caution about the time of day you perform a DBCC CHECKDB operation. Other functions can also affect the server and temporarily increase system overhead.
Note
DBCC DBREPAIR was never used often, and has been removed from the product. It is recommended that you use DROP DATABASE to drop damaged databases. DBCC DBREPAIR is no longer supported.
DBCC Maintenance Operations
Regular maintenance is
needed in all database environments. Data and index pages become
fragmented. Data may become corrupt, and file sizes may need to be
adjusted. Regular maintenance helps you optimize the server environment.
The DBCC options that you use in maintenance processes are as follows:
DBCC DBREINDEX—Rebuilds one or more indexes.
DBCC INDEXDEFRAG—Defragments clustered and secondary indexes of the specified table or view.
DBCC SHRINKDATABASE—Shrinks the size of the data files in the specified database.
DBCC SHRINKFILE—Shrinks a specified data file or log file.
DBCC UPDATEUSAGE—Reports and corrects inaccuracies in the sysindexes table, which may result in incorrect space use reported by sp_spaceused.
Miscellaneous DBCC Operations
Some DBCC
options do not directly fit into any of the categories . Listed as miscellaneous options, these DBCC operations can provide assistance, help to free and better use resources, and provide some tracking mechanisms:
DBCC dllname (FREE)—Unloads the specified extended stored procedure DLL from memory.
DBCC HELP—Returns syntax information for the specified DBCC statement.
DBCC PINTABLE—Marks a table to be pinned and does not flush the pages for the table from memory.
DBCC ROWLOCK—Is used for Microsoft SQL Server 6.5, enabling insert row locking operations on tables.
DBCC TRACEOFF—Disables trace flags. (Trace flags are discussed in the next section.)
DBCC TRACEON—Enables trace flags. (Trace flags are discussed completely in the next section.)
DBCC UNPINTABLE—Marks a table as unpinned. Table pages in the buffer cache can be flushed.
Note
The DBCC ROWLOCK
option is somewhat antiquated. Row-level locking is enabled by default
in SQL Server 2005, where the locking strategy is row locking with
possible promotion to page or table locking. DBCC ROWLOCK is included in SQL Server 2005 for backward compatibility. In a future version of SQL Server, DBCC ROWLOCK may not be supported.
Setting Alerts to Automate Problem Identification
SQL Server
Agent can monitor performance conditions and fire alerts when specific
thresholds are met. This can help in notifying an administrator when
disk space is running low or when applications are unresponsive. You can
also set up alerts to fire based on criteria within the data. You are
not likely to run into this on the 70-431 exam, but it is a useful
feature of SQL Server.
