COAX EVEN MORE SPEEDOUT OF YOUR SQL SERVER DATABASES

Microsoft's SQL Server lets you build powerful and reliable database applications quickly, but making those applications perform their best can be tricky. Luckily, database administrators and developers can use several simple techniques to coax more speed out of a SQL Server database. In October 1996 ("10 Easy Tips for Better SQL Server Performance"), I discussed some performance-boosting tactics. Here are 10 more ideas that you can easily apply to your SQL Server database applications.

Tip 1:
Use Stored Procedures Where Possible

You can use procedural language (Transact-SQL) and SQL to create functions that are stored in the database engine, instead of in application code or libraries. Such stored procedures have several advantages. These procedures eliminate runtime parsing, because SQL parses them when you create them. You can designate certain stored procedures to run with database administrator (DBA) privileges, even if the user running them does not have this level of security. This feature lets you combine a high level of data access with tight security. With stored procedures, you can easily create libraries of functions, thereby reducing the amount of original code that your programmers must write. Stored procedures greatly reduce the amount of work needed to perform an upgrade because you can move application logic onto the server instead of distributing new versions of software to each client in an organization. When you store procedures, the SQL Server engine can read the procedures from the memory buffer instead of from the disk, thus reducing the overall amount of costly disk I/O operations. Finally, in a distributed environment, stored procedures let you cut the amount of information traveling between the front end (client) and the back end (server). This reduction can save time, especially if client and server are far apart. Another way to reduce traffic between the client and server during stored procedures is to set the NOCOUNT option. NOCOUNT disables SQL Server's DONE_IN_PROC messages, which show the number of rows a particular operation affects.

Tip 2:
Select the Best Read-Ahead Configuration Values

One SQL Server feature that can dramatically improve query performance is read-ahead, (RA, or parallel data scan). When SQL Server detects certain queries, such as table scans and other searches that return large quantities of data sequentially, it allocates a background thread to read ahead in the table. The result is that by the time your program requests this information, SQL Server might have already fetched the data into the buffer pool.

For example, suppose you're running a long report that pulls information from a large customer table. If you're reading large blocks of data sequentially, SQL Server can anticipate the next series of information that you want and read these rows into memory while you're still processing the first batch of data. This action can lead to substantial performance gains, because your program may now be able to find what it needs in memory, rather than on disk.

Let's look at how you can set the parameters listed in the SQL Server Configuration/Options dialog box to best take advantage of RA. Don't forget that changes to any RA parameters will affect database access for all SQL Server-based applications running on your system. Therefore, make changes to these parameters carefully, because an alteration can produce unintended results.

RA cache hit limit. Sometimes the RA manager attempts to locate data from the disk but finds the data still in the buffer pool. In this case, RA is not very effective. You can set the hit-limit parameter to restrict the number of buffer pool hits that an RA query encounters before the manager abandons the RA strategy. The valid range of values is between 1 and 255 hits; the default value is 4. Do not set the value excessively high; if the RA manager finds pages already in the buffer pool, the manager has no reason to continue reading ahead.

RA cache miss limit. SQL Server uses the RA cache miss limit to determine when to start reading ahead. For example, if you've set the RA cache miss limit to 5, SQL Server starts reading ahead after not finding five pages in the buffer pool. The valid range of values is between 1 and 255; the default is 3.

Setting this value too low means that SQL Server tries to read ahead on most queries; setting the value too high causes SQL Server to avoid a potentially beneficial strategy. So, if you use your system primarily for reporting and other operations that usually fetch large batches of information, set the value on the low side.

Setting it to 1, however, means that SQL Server will always issue an RA request, even when retrieving only one data page from disk. This process will negatively affect performance in most cases. This setting tells SQL Server that you usually want to start RA operations as quickly as possible. Conversely, if your system works as an OnLine Transaction Processing (OLTP) environment, with very few batch, sequential operations, raise this value because you want SQL Server to avoid RA in all but the most obvious situations.

RA delay. SQL Server uses the RA delay parameter to determine how long to wait before starting to read ahead. This value is necessary because some time always elapses between when the RA manager starts and when it can service requests. The valid range of values is between 0 and 500 milliseconds; the default is 15. The default suffices for most environments, but if you're running SQL Server on a multiprocessor machine, set it lower. Setting this parameter too high means that SQL Server can delay too long before embarking on an RA.

RA pre-fetches. You can use the RA pre-fetches measure to tell SQL Server how many extents you want it to pre-fetch during RA operations. The valid range of values is between 1 and 1000, with a default value of 3. If your applications perform primarily large sequential operations, set this value to a higher number to tell SQL Server to bring larger amounts of data into the buffer pool during each RA operation. If you set this number too high, however, you can displace other users' buffer pool pages with your data. Consequently, be careful when you experiment with this number; increase the value gradually. Try raising the value by 5 percent each time, and keep a record of overall system response between changes. Find out whether performance gains for one application degrade the performance of other applications.

RA worker threads. Threads process RA operations. The RA-slots-per-thread parameter controls the number of threads that SQL Server allocates to service RA requests. Each configured thread then supports a number of individual RA requests. The RA worker thread setting can range between 0 and 255; the default is 3. Set this option to the maximum number of concurrent users that you expect to access SQL Server. If you set this parameter too low, you might not have enough threads to service the volume of RA requests. If you set it too high, you'll start too many RA threads. SQL Server logs an error if the number of RA worker threads exceeds the number of RA slots.

RA slots per thread. The RA-slots-per-thread parameter configures the number of RA requests that each thread manages. The valid range is between 1 and 255 operations; the default is 5. If you set this value too high, SQL can overload your RA threads; the thread can spend more time switching between different RA requests than servicing the requests. A low value can lead to idle threads. Usually, the default is fine.

One final note regarding tuning RA parameters. Do not experiment with these numbers until you are comfortable with both the architecture of SQL Server and the specific performance characteristics of your system. Even when you decide to experiment, remember to change only one parameter at a time. Changing multiple parameters at once degrades performance without giving you much information about why response has become worse.

Tip 3:
Use Wide Indexes Correctly

A wide index is one that contains a large amount of data--either many small columns or several big columns. For example, an index of one char(255) column is wider than an index of five char(4) columns.

Narrow indexes are generally better. When you create an index, SQL Server stores the index's key values and record locations on index pages. With narrow indexes, SQL Server can fit more key values and pointers on each index page. This structure lets the optimizer find your data more quickly, because it has to read fewer index pages before encountering your data. In addition, when more index keys and pointers are on a page, the optimizer can churn through this information more effectively. Conversely, when your index keys are wide, the engine can fit only a few key value and data pointers on each page. The index structure also tends to be deeper when keys are wide, so the optimizer has to carry out even more computations.

In the Customer_master table in Table 1, suppose you place a composite index on the last_name, first_name, street, and city columns. This index is wide, because it contains a relatively high number of columns and a great deal of data.

TABLE 1: Customer_master
account_number last_name first_name street city ... account_balance
344484454 Bockwinkle Terry Jeeves Way 24998
344484455 Okerlund Nick Jacques St. 105660
344484456 Blassie George Mariner Rd. 3004
. . . . . .
. . . . . .
. . . . . .

You need to analyze why you create wide indexes. Do the users want to search on all these fields together; do they want to sort on all these fields? More likely, this index represents a kitchen sink approach, whereby you put as many columns in the index as possible.

This approach isn't as effective as you might think. The performance obstacle becomes painfully obvious when you ask the optimizer to search on only one or two of the index's columns. In Table 1, if you want to find all rows that have a last_name value between Hank and Hendrix, the optimizer has to use the composite index because no other indexes exist. Unfortunately, the optimizer now needs to read potentially hundreds of index pages to locate the appropriate information because the index's width means that the index is also deep. However, if you have one index on last_name, you can fit more keys on each index page, and the optimizer can find the correct information more quickly. A wide index also can cause your sort order to force the engine into a sequential scan, in spite of the index's existence, because of the way SQL Server uses a composite index to satisfy searches and sorts.

Despite these warnings, wide indexes are effective if you request only columns from a particular nonclustered index. (In a nonclustered index, the engine does not reorder the table data to match the index.) SQL Server can bypass reading the data and get its information from the index keys. For example, the query

select last_name, first_name,
from customer_master
where last_name between Zlotnick and Zotnick

asks for only the last_name and first_
name columns from the customer_
master table. SQL Server can satisfy this request without reading the table's data pages, because both columns are in the index and the index is nonclustered. If you retrieve large blocks of data, this shortcut can improve performance, especially because disk I/O can add a great deal of overhead to a query. So, wide indexes are not always performance-wreckers.

Tip 4:
Determine the Right Size for the Transaction Log

To correctly size your system's transaction log, start by allocating about 15 percent to 25 percent of your total database disk storage to the transaction log. Then consider some factors that affect usage of the transaction log.

An application that performs primarily read-only data access is not likely to require a large transaction log, because the application uses the log only when it modifies information. Conversely, if your application performs millions of modifications each day, you can count on needing a larger transaction log. However, if your application performs frequent but small data modifications, you might not need an enormous transaction log.

The recovery interval parameter helps control how often checkpoints occur. During a checkpoint, SQL Server synchronizes the contents of the transaction log and the disk. In most cases, the longer the interval between checkpoints, the larger the transaction log you need.

If you infrequently save the transaction log to media, instead of automatically truncating the log, you need to create a larger transaction log. Saving the log frequently is a tactic that lets you avoid creating a large transaction log. Also, remember that because you can't restore transactions after you truncate the log, automatically truncating the transaction log means that you're willing to lose transactions between data backups if your system fails.

If you use the CREATE DATABASE statement, specify log size with the following syntax:

\[LOG ON database_device
\[= size\]
\[, database_device
\[= size\]\]...\]

If you use the SQL Enterprise Manager, specify log size with the dialog box shown in Screen 1.

If the data and the transaction log are on the same database device, you can leave the transaction log size value blank, because the transaction log will consume only as much space as necessary. However, for most production systems, place the transaction log on a separate device. For this example, assume that you use different database devices to hold your data and transaction log.

Start with a small transaction log. You can easily add space if you find that you've made the transaction log too small, but you can't make the log smaller without a great deal of work.

You can increase the size in two ways: First, you can use the SQL Enterprise Manager to expand the database screen. This screen lets you pick a preallocated device and then assign extra storage to the transaction log. The other way to increase the size of the transaction log is to use the ALTER DATABASE command and sp_logdevice stored procedure. For example, if you want to add 50MB to a database called acquisition, the syntax is

alter database acquisition on logdevice5 = 50
sp_logdevice acquisition, logdevice5

If you increase the transaction log's size, back up the master database before and after the alteration.

Tip 5:
Put TEMPDB in RAM

In certain circumstances, you can improve system throughput by storing the TEMP database (TEMPDB) in RAM. This database is where SQL Server builds temporary tables and performs much of its internal sorting.

First, this technique is appropriate only when your system has enough memory to meet SQL Server's basic caching needs. If your system doesn't have enough memory to begin with, taking more for TEMPDB can decrease overall performance. Another time when having TEMPDB in RAM is beneficial is when your TEMPDB operations fit into the space you've allocated. For example, if you've allocated 2MB for TEMPDB RAM and each instance of your application routinely creates 10MB work tables, having TEMPDB in RAM won't make much difference, because you won't have enough space to satisfy all user demands.

Placing TEMPDB in RAM improves performance when your users and applications use TEMPDB heavily. If you don't access TEMPDB often, placing it into RAM may worsen performance, because TEMPDB now occupies valuable RAM storage.

You can tell how heavily you're using TEMPDB by running the SHOWPLAN command against your queries. If you see frequent implicit work tables, chances are you're hitting TEMPDB quite often. But if most of your queries don't require the engine to create work tables, storing TEMPDB in RAM probably isn't necessary.

Another circumstance in which placing TEMPDB in RAM can help performance is when your applications don't access cached data frequently; in these cases, your programs are continually accessing the disk drive to locate data, rather than finding it in memory. For example, in applications where individual users are looking at vastly different data records, one user has little chance of finding data that another user has already cached.

All in all, using RAM for caching data and index pages is probably better than placing TEMPDB in RAM. However, if you decide to place TEMPDB in RAM, record your performance before and after you put TEMPDB in RAM; if you don't experience significantly better throughput, keep your TEMPDB on disk.

To get the best performance from storing TEMPDB in RAM, restart the engine after changing the storage. If you change it while the engine is running, SQL Server may use noncontiguous memory to satisfy your request. When you restart the engine, TEMPDB's memory will be coterminous. Via Perfmon, SQL Server 6.5 now lets you track the maximum amount of used storage in your TEMPDB.

Tip 6:
Avoid Transactions for Certain Operations

Use transactions only if your program modifies the database. Don't use transactions for queries, reports, work tables, or bulk operations.

If you're running a query and it doesn't have any INSERT/UPDATE/
DELETE statements, you can omit transactions, either by selecting this option from your application development tool or by not issuing a BEGIN TRANSACTION statement. Many programmers open a transaction at the start of a report, but running a report inside a transaction doesn't contribute anything to the report (unless you are also updating tables). An open transaction during a report can even slow your system's performance.

Sometimes you need to create temporary work tables, but the traditional concepts of database integrity and transactional control might not apply when you're using work tables. Thus, you can often avoid using transactions when populating your work tables or modifying information.

Bulk operations are procedures that make substantial changes to database tables. Often, you don't need to record these events in the transaction log. You can disable transactions for bulk operations by setting a database configuration flag that bypasses transactions during bulk inserts and SELECT INTO statements. Or you can modify your application code to omit transactions when you perform a bulk operation.

You sometimes have to use transactions, however, to avoid unwanted data modifications during an operation. For example, during a long query or report, you often want to freeze the underlying data until the report has finished. To do so, you may need to request that the optimizer lock all rows until the operation has finished; locking rows requires that you begin a transaction.

Tip 7:
Allocate the Correct Amount of Memory for Stored Procedure Caching

Memory caching is a key component of SQL Server's architecture. The memory cache is divided between storage for data (the data cache) and storage for stored procedures (the stored procedure buffer). Just as SQL Server uses the data cache to reduce the amount of disk I/O necessary to retrieve data, SQL Server uses the stored procedure buffer to locate procedures in memory, rather than reading them from disk.

When you try to execute a previously built stored procedure, SQL Server first looks in the procedure cache to see whether the procedure is already in memory. If it is, the engine uses the memory-based version of the stored procedure. If not, the engine reads the procedure from disk and places it in the stored procedure buffer, consuming as many 2KB memory pages as necessary. When you create or compile a stored procedure, SQL Server also uses the stored procedure buffer to cache this information for later users. However, the engine doesn't support multiple users working with the same query plan at the same time; therefore, stored procedures are reusable, not re-entrant.

You allocate the total amount of memory for the engine by setting the Memory parameter. After SQL Server has started and defined all necessary internal memory structures, it assigns leftover memory to the stored procedure cache and the data cache.

The Procedure Cache parameter tells the engine what percentage of this leftover memory to assign to the procedure cache; the remainder goes to the data cache. The default value for the Procedure Cache parameter is 30 percent. You can raise or lower this value according to how much you think your application will use stored procedures.

You can use the statistics from the SQL Server Procedure Cache object to monitor stored procedure cache activity. Pay particular attention to the maximum value parameters, such as

Maximum procedure buffers active percentage
Maximum procedure buffers used percentage
Maximum procedure cache active percentage
Maximum procedure cache used percentage

These values represent the high-water marks since you last started the engine.

Tip 8:
Use Joins Instead of Sequential SELECT Statements

TABLE 2: Personnel
personnel_id first_name last_name date_of_hire...
805 Sylvia Migdal Aug 2 1984
. . . .
. . . .
. . . .
1108 Earl O'Snyder Nov 1 1992
1109 Robert Gordon Nov 8 1992
TABLE 3: Sales_calls
call_id personnel_id date_of_call company_name...
1309 805 Jan 30 1997 Hasenffefer Corp.
. . . .
. . . .
. . . .
2709 1509 Aug 15 1997 Freedom Rider, Inc.

Many programmers, especially those with backgrounds in mainframe COBOL, dBASE, or indexed sequential access method (ISAM) files, avoid joining between tables and instead process database records sequentially. This approach doesn't use the full power of the database and the SQL programming language.

For example, you are developing a system to track sales calls by your company's representatives, and you want to produce a report that shows all sales calls an employee has made between January 1 and December 31, 1997. To write the report, you work with two tables, Personnel (Table 2) and Sales_calls (Table 3). The Personnel table contains information about each representative, and the Sales_calls table tracks individual sales call statistics. Your output from the report must look like this:

Employee number: nnnnn
Employee name : last, first
Sales call date Company name...
--------------------------------
mm/dd/yyyy xxxxxxxxxxxxxx
mm/dd/yyyy xxxxxxxxxxxxxx

If you process the report without joins, your pseudocode looks like

declare cursor to get records from personnel table
open cursor
foreach cursor into host variable
structure (personnel)
print employee name and number
and header info
declare cursor to get records
from sales_calls table
open cursor using the contents
of personnel host variable as key into sales_calls table
foreach cursor into host
variable structure_
(sales_calls)
print sales call information
end foreach
close cursor
end foreach
close cursor

This process contains extra engine work. For every employee found in the Personnel table, you declare, open, and close a cursor to get data from the Sales_calls table. If you have thousands of employees, a better approach is to let the database engine join records between the Personnel and Sales_calls tables. You can do this by using the personnel_id column as a foreign key. Your new pseudocode looks like this:

declare a cursor to get records from personnel and sales_calls tables
open cursor
foreach cursor into host variable structure (personnel + sales_calls)
print employee name, number
and header info_
(only once)
print sales call information
end foreach
close cursor

Alternatively, you can use a server-based stored procedure and return only appropriate rows to the client.

If you want the report to include employees who made no sales calls during this period, use a SQL join-based method. Instead of performing an ordinary join between the tables, use an OUTER join, with the Personnel table as primary and the Sales_calls table as outer.

select *
frompersonnel,outersales_calls
where personnel.personnel_id = sales_calls.personnel_id

By joining between tables, you take better advantage of database engine performance features.

Tip 9:
Don't Create Too Many Indexes

Some database administrators try to anticipate every possible sort and search combination by creating indexes on practically every column in every table. Having too many indexes impedes your system in many ways. Every time you perform an insert or delete, you have to modify the indexes and the data. When you update an indexed column, the SQL Server engine updates all affected indexes, an action that can have the added undesirable effect of causing the engine to restructure the index trees. This update operation can impede performance for all applications accessing the table and can even briefly degrade response across the entire system. You have no way of knowing whether the engine has restructured the index trees. Extra indexes also consume more disk space. Finally, when confronted with too many indexes, the optimizer may not choose the best-qualified index. Your database operation may run more slowly than if you had fewer indexes.

The best way to know whether you have too many indexes is to test your database operations with SHOWPLAN. Simulate a typical work day, remove the SHOWPLAN command from any procedures or code that you modified, and then review the output. You can quickly determine which index SQL Server is using and then remove any indexes that the engine doesn't reference often.

Sometimes you need additional indexes to handle specific, easily identifiable tasks, such as an end-of-month processing suite. In these cases, create the indexes immediately before you need them and drop them as soon as you finish. At other times, you need to run large batch update operations, which can be time-consuming if you have too many indexes to update. You can benefit from creating a stored procedure to drop some indexes, perform the operation, and then re-create the indexes. The overall time to do this can be less than if you let the batch update operation alter the extra indexes.

Tip 10:
Use the Multiple Table DELETE Option

Traditional SQL limits your delete operations to one table at a time. Transact-SQL has a multi-table delete capability that reduces the number of individual engine calls. For example, to delete rows in two tables, resources and parts, you can issue two SQL statements:

delete from resources where resource_cost > 5000
delete from parts where part_cost > 5000
Or you can use Transact-SQL's multiple table DELETE extension:
delete from resources
from parts
where resources.resource_cost = parts.part_cost
and resources.resource_cost > 5000

This approach is not portable, so you can't run your application against different databases. But if you work only with SQL Server, multi-table is a convenient shortcut. You can also use the UPDATE statement to alter several tables at one time.

A Last Word
As you experiment with these tips, change only one parameter at a time so you know which change produces which effect. And be sure you back up your database before and after each change.

For more information about these tips and other suggestions for enhancing SQL Server's performance, read my book, Microsoft SQL Server: Planning and Building a High-Performance Database.

Microsoft SQL Server : Planning and Building a High-Performance Database
Author: Robert D. Schneider
Publisher: Prentice Hall PTR
Upper Saddle River, N.J., 1997