The company I work for is a web development company that uses SQL Server for its databases. The company is not quite big enough to have a DBA, so each of the developers takes a varying amount of responsibility to manage the databases. Over time, I have gravitated perhaps more than the others toward this responsibility and have learned several things that have really helped to maintain them well. If you are in the same position, I hope these things can be a benefit to you as well.
1. Index Fragmentation
Over time, indexes inevitably get fragmented and need to be rebuilt. A table with GUID’s that are not sequential will fragment really quickly if there are a good amount of inserts regularly. So, how do you know if you need to rebuilding your indexes? Fortunately, you can run a simple query to get this information. It should be noted that you need not worry about fragmentation on indexes that are very small. The following query gets the fragmentation statistics of indexes with a (memory) page count greater than 50 and a fragmentation greater than 25 percent. (Both of these variables may be adjusted.)
SELECT sys.objects.name AS [Table Name], sys.indexes.name AS [Index Name], partition_number AS [Partition], CAST(avg_fragmentation_in_percent AS DECIMAL (4,2)) AS [Fragmentation Percentage], CONVERT(decimal(18,2), page_count * 8 / 1024.0) AS [Total Index Size (MB)], page_count AS [Page Count] FROM sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL , NULL, 'LIMITED') IndexStats INNER JOIN sys.objects ON sys.objects.object_id = IndexStats.object_id INNER JOIN SYS.indexes ON SYS.indexes.object_id = SYS.OBJECTS.object_id AND IndexStats.index_id = sys.indexes.index_id WHERE avg_fragmentation_in_percent > 25 AND Indexes.index_id > 0 AND page_count > 50
So, how did it look? If you have index fragmentation percentages over 90 for indexes that use a lot of pages, perhaps more than 500, then you probably need to set up a SQL Agent job to run weekly to rebuild the indexes. For more information on how to set this up, see SQL Server Optimization Script with Index Rebuilding to Reduce Fragmentation
2. Identifying and Optimizing Expensive Queries
If your database has never been analyzed or profiled, there may be some queries that are really inefficient and may be impacting your overall site performance. Oftentimes, when new tables are created, indexes are added that are thought to be necessary. However, only real world experiences will reveal if an additional index is needed or if a query needs to be rewritten to leverage indexes that are already built. To identify expensive queries, use the SQL Server Profiler tool from the Tools menu in SQL Server Management Studio.
For starters, I would suggest running it for five minutes, setting a threshold to ignore queries with less than 25 reads. Make sure you write the results to a database table. That way you can run queries on them and perform some careful analysis. Oftentimes, adding the right indexes or restructuring a few queries can drastically reduce the load on your database.
For more details on how to do this, see SQL Server Database Optimization ? a Beginner?s Guide.
3. Running Reports to Identify Largest Tables and Indexes
Do you know what your largest tables are? Do you know which table requires the most space for its indexes? Well, it would be really good to know both of those things, and it’s pretty easy to find that information. Right-click on your database, scroll down to “Reports”, then “Standard Report”, and then “Disk Usage by Top Tables”. This will provide you with some great information. It comes initially sorted by the amount of reserved space used per table. This is a combination of the space for the data and the indexes combined. You also can see the number of records in each table and the amount of unused space.
Knowing which of your tables is the largest and which table requires the most space for its indexes can help direct your efforts in analyzing how your database. Sometimes you can find an index that is really large but is unnecessary. I found a column in a table that was very large, but was no longer being used or referenced anywhere in our system. I was able to simply set all the values of the column to blanks and recoup quite a bit of space, and, again, this was for data that we were not using at all anymore. This will often highly log tables that contain data that is no longer necessary as well. See later in this post on removing such tables.
4. Cleaning up log tables
Oftentimes a database will have tables for errors, actions, emails, and various other things like that. However, it is rarely necessary to store all of this data in the database forever. An entry in the error log may be of interest for a month or two, but after that amount of time has passed, it has either been resolved or was not important enough to deal with. Furthermore, as the database changes, it becomes more and more difficult to really recreate what caused the problem.
If you have a good database backup plan–keeping copies of daily, weekly, monthly, and yearly backups, then you can probably remove old rows. If you keep every yearly backup, then should be fine to remove all rows older than one year. The January 1st backup will contain all of the errors for the previous year, so removing older rows only removes them from the current version of the database. You can still get back to older rows by restoring annual backups if your really need to do so.
For other tables, such as action logs, it may be necessary to be able to go back as much as a year, but if there is no business reason for being able to go back farther than that, you can probably remove rows older than perhaps two months. Your monthly backups will have a record that you can retrieve if necessary.
These types of tables can grow to be fairly large over time. If the tables are indexed, then your index rebuilding jobs will have to deal with them and basic inserts will become slower as well. Consequently, taking a careful look at tables like these and then scheduling a monthly SQL Agent job to remove old rows can really serve to keep your database lean and efficient.
5. Identifying and Removing Unused Indexes
There is probably a good chance that you have some indexes in your database that are not being used. These unused indexes can sometimes be rather large. This results in increased database size and backup storage, decreased insert time, and increased time to rebuild the indexes when they get too fragmented. So, it’s probably good to run a query to see if you have any unused indexes that are medium to large. You probably don’t need to worry about small indexes that are not currently being used.
There are actually two queries that you can run. There is an index usage table that you can reference and see when certain values are set to zero. Those indexes are not being used. However, there are also some indexes that, for some reason, never seem to even show up in that table. So, you have to run two different queries:
Indexes Not In Index Usage Table
-- GET UNUSED INDEXES THAT DO **NOT** APPEAR IN THE INDEX USAGE STATS TABLE DECLARE @dbid INT SELECT @dbid = DB_ID(DB_NAME()) SELECT Databases.Name AS [Database], object_name(Indexes.object_id) AS [Table], Indexes.NAME AS [Index], Indexes.INDEX_ID, PhysicalStats.page_count as [Page Count], CONVERT(decimal(18,2), PhysicalStats.page_count * 8 / 1024.0) AS [Total Size (MB)], CONVERT(decimal(18,2), PhysicalStats.avg_fragmentation_in_percent) AS [Frag. %] FROM SYS.INDEXES Indexes INNER JOIN SYS.OBJECTS Objects ON Indexes.OBJECT_ID = Objects.OBJECT_ID LEFT JOIN sys.dm_db_index_physical_stats(@dbid, null, null, null, null) PhysicalStats ON PhysicalStats.object_id = Indexes.object_id AND PhysicalStats.index_id = indexes.index_id INNER JOIN sys.databases Databases ON Databases.database_id = PhysicalStats.database_id WHERE Objects.type = 'U' -- User Table AND Indexes.is_primary_key = 0 AND Indexes.type = 2 -- Nonclustered indexes AND Indexes.INDEX_ID NOT IN ( SELECT UsageStats.INDEX_ID FROM SYS.DM_DB_INDEX_USAGE_STATS UsageStats WHERE UsageStats.OBJECT_ID = Indexes.OBJECT_ID AND Indexes.INDEX_ID = UsageStats.INDEX_ID AND DATABASE_ID = @dbid)
Unused Indexes In the Index Usage Table
-- GET UNUSED INDEXES THAT APPEAR IN THE INDEX USAGE STATS TABLE DECLARE @MinimumPageCount int SET @MinimumPageCount = 500 SELECT Databases.name AS [Database], object_name(Indexes.object_id) AS [Table], Indexes.name AS [Index], PhysicalStats.page_count as [Page Count], CONVERT(decimal(18,2), PhysicalStats.page_count * 8 / 1024.0) AS [Total Size (MB)], CONVERT(decimal(18,2), PhysicalStats.avg_fragmentation_in_percent) AS [Frag. (%)], ParititionStats.row_count AS [Row Count], CONVERT(decimal(18,2), (PhysicalStats.page_count * 8.0 * 1024) / ParititionStats.row_count) AS [Index Size/Row (Bytes)], UsageStats.last_user_scan, UsageStats.last_user_seek FROM sys.dm_db_index_usage_stats UsageStats INNER JOIN sys.indexes Indexes ON Indexes.index_id = UsageStats.index_id AND Indexes.object_id = UsageStats.object_id INNER JOIN SYS.databases Databases ON Databases.database_id = UsageStats.database_id INNER JOIN sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL, NULL, NULL) AS PhysicalStats ON PhysicalStats.index_id = UsageStats.Index_id AND PhysicalStats.object_id = UsageStats.object_id INNER JOIN SYS.dm_db_partition_stats ParititionStats ON ParititionStats.index_id = UsageStats.index_id AND ParititionStats.object_id = UsageStats.object_id WHERE ((UsageStats.user_scans = 0 AND UsageStats.user_seeks = 0) OR (UsageStats.last_user_scan AND (UsageStats.last_user_seek -- ignore indexes with less than a certain number of pages of memory AND PhysicalStats.page_count > @MinimumPageCount -- Exclude primary keys, which should not be removed AND Indexes.is_primary_key = 0
For more information on this, see Identifying Unused Indexes in a SQL Server Database
6. Identifying and Removing Duplicate or Near Duplicate Indexes
In addition to removing unused indexes, you can also remove duplicate indexes or near-duplicate indexes. Obviously, a duplicate index is unnecessary, but sometimes a near-duplicate index can be removed without any adverse effects. I have found two indexes that used all of the same columns in the index itself and only differed in the columns that were “included” in the index (but not indexed). If there are two indexes that only differ by one included column and the index is large, then you may be able to remove the simpler index. Of course, it’s a good practice to save a script that can quickly regenerate the index if you find that it is actually necessary after all.
Ok, so here’s the query:
WITH IndexSummary AS ( SELECT DISTINCT sys.objects.name AS [Table Name], sys.indexes.name AS [Index Name], SUBSTRING((SELECT ', ' + sys.columns.Name as [text()] FROM sys.columns INNER JOIN sys.index_columns ON sys.index_columns.column_id = sys.columns.column_id AND sys.index_columns.object_id = sys.columns.object_id WHERE sys.index_columns.index_id = sys.indexes.index_id AND sys.index_columns.object_id = sys.indexes.object_id AND sys.index_columns.is_included_column = 0 0RDER BY sys.columns.name FOR XML Path('')), 2, 10000) AS [Indexed Column Names], ISNULL(SUBSTRING((SELECT ', ' + sys.columns.Name as [text()] FROM sys.columns INNER JOIN sys.index_columns ON sys.index_columns.column_id = sys.columns.column_id AND sys.index_columns.object_id = sys.columns.object_id WHERE sys.index_columns.index_id = sys.indexes.index_id AND sys.index_columns.object_id = sys.indexes.object_id AND sys.index_columns.is_included_column = 1 0RDER BY sys.columns.name FOR XML Path('')), 2, 10000), '') AS [Included Column Names], sys.indexes.index_id, sys.indexes.object_id FROM sys.indexes INNER JOIN SYS.index_columns ON sys.indexes.index_id = SYS.index_columns.index_id AND sys.indexes.object_id = sys.index_columns.object_id INNER JOIN sys.objects ON sys.OBJECTS.object_id = SYS.indexES.object_id WHERE sys.objects.type = 'U' ) SELECT IndexSummary.[Table Name], IndexSummary.[Index Name], IndexSummary.[Indexed Column Names], IndexSummary.[Included Column Names], PhysicalStats.page_count as [Page Count], CONVERT(decimal(18,2), PhysicalStats.page_count * 8 / 1024.0) AS [Size (MB)], CONVERT(decimal(18,2), PhysicalStats.avg_fragmentation_in_percent) AS [Fragment %] FROM IndexSummary INNER JOIN sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL, NULL, NULL) AS PhysicalStats ON PhysicalStats.index_id = IndexSummary.index_id AND PhysicalStats.object_id = IndexSummary.object_id WHERE (SELECT COUNT(*) as Computed FROM IndexSummary Summary2 WHERE Summary2.[Table Name] = IndexSummary.[Table Name] AND Summary2.[Indexed Cols] = IndexSummary.[Indexed Cols]) > 1 0RDER BY [Table Name], [Index Name], [Indexed Column Names], [Included Column Names]
For more information on this, see Query to Identify Duplicate or Redundant Indexes in SQL Server
7. Checking Identity Columns for Overflow Risk
If your database is using integer identity columns, then the largest ID that can be stored is 2,147,483,647 (2.1 billion). Most tables never come close to overflowing, but it is definitely better to make sure than to not be sure and experience an overflow one day. So, here’s an easy query to quickly check:
SELECT sys.tables.name AS [Table Name], last_value AS [Last Value], CASE (MAX_LENGTH) WHEN 1 THEN 'TINYINT' WHEN 2 THEN 'SMALLINT' WHEN 4 THEN 'INT' WHEN 8 THEN 'BIGINT' ELSE 'UNKNOWN' END AS DataType, CAST(cast(last_value as int) / 2147483647.0 * 100.0 AS DECIMAL(5,2)) AS [Percentage of ID's Used] FROM sys.identity_columns INNER JOIN sys.tables ON sys.identity_columns.object_id = sys.tables.object_id
For more information, see Checking Integer Identity Columns in SQL Server for Overflow Risk
Alright, so there are some tips that I hope you will find helpful. Good luck!