Chapter 9. System Catalog. In This Chapter. c Introduction to the. System Catalog. c General Interfaces c Proprietary Interfaces

Transcription

1 Chapter 9 System Catalog In This Chapter c Introduction to the System Catalog c General Interfaces c Proprietary Interfaces

2 260 Microsoft SQL Server 2012: A Beginner s Guide This chapter discusses the system catalog of the Database Engine. The introduction is followed by a description of the structure of several catalog views, each of which allows you to retrieve metadata. The use of dynamic management views and dynamic management functions is also covered in the first part of the chapter. Four alternative ways for retrieving metadata information are discussed in the second part: system stored procedures, system functions, property functions, and the information schema. Introduction to the System Catalog The system catalog consists of tables describing the structure of objects such as databases, base tables, views, and indices. (These tables are called system base tables.) The Database Engine frequently accesses the system catalog for information that is essential for the system to function properly. The Database Engine distinguishes the system base tables of the master database from those of a particular user-defined database. System tables of the master database belong to the system catalog, while system tables of a particular database form the database catalog. Therefore, system base tables occur only once in the entire system (if they belong exclusively to the master database), while others occur once in each database, including the master database. In all relational database systems, system base tables have the same logical structure as base tables. As a result, the same Transact-SQL statements used to retrieve information in the base tables can also be used to retrieve information in system base tables. The system base tables cannot be accessed directly: you have to use existing interfaces to query the information from the system catalog. There are several different interfaces that you can use to access the information in the system base tables: Catalog views Present the primary interface to the metadata stored in system base tables. (Metadata is data that describes the attributes of objects in a database system.) Dynamic management views (DMVs) and functions (DMFs) Generally used to observe active processes and the contents of the memory.

3 Chapter 9: System Catalog 261 Information schema A standardized solution for the access of metadata that gives you a general interface not only for the Database Engine, but for all existing relational database systems (assuming that the system supports the information schema). System and property functions Allow you to retrieve system information. The difference between these two function types is mainly in their structure. Also, property functions can return more information than system functions. System stored procedures Some system stored procedures can be used to access and modify the content of the system base tables. Figure 9-1 shows a simplified form of the Database Engine s system information and different interfaces that you can use to access it. This chapter shows you just an overview of the system catalog and the ways in which you can access metadata. Particular catalog views, as well as all other interfaces, that are specific for different topics (such as indices, security, etc.) are discussed in the corresponding chapters. These interfaces can be grouped in two groups: general interfaces (catalog views, DMVs and DMFs, and the information schema), and proprietary interfaces in relation to the Database Engine (system stored procedures and system and property functions). Catalog views System procedures Information schema DMVs and DMFs System and property functions System base tables System information Figure 9-1 Graphical presentation of different interfaces for the system catalog

4 262 Microsoft SQL Server 2012: A Beginner s Guide General means that all relational database systems support such interfaces, but use different terminology. For instance, in Oracle s terminology, catalog views and DMVs are called data dictionary views and V$ views, respectively. The following section describes general interfaces. Proprietary interfaces are discussed later in the chapter. General Interfaces As already stated, the following interfaces are general interfaces: Catalog views DMVs and DMFs Information schema Catalog Views Catalog views are the most general interface to the metadata and provide the most efficient way to obtain customized forms of this information (see Examples 9.1 through 9.3). Catalog views belong to the sys schema, so you have to use the schema name when you access one of the objects. This section describes the three most important catalog views: sys.objects sys.columns sys.database_principals You can find the description of other views either in different chapters of this book or in Books Online. The sys.objects catalog view contains a row for each user-defined object in relation to the user s schema. There are two other catalog views that show similar information: sys.system_objects and sys.all_objects. The former contains a row for each system object, while the latter shows the union of all schema-scoped user-defined objects and

5 Chapter 9: System Catalog 263 Column Name name object_id schema_id type Description Object name Object identification number, unique within a database ID of the schema in which the object is contained Object type Table 9-1 Selected Columns of the sys.objects Catalog View system objects. (All three catalog views have the same structure.) Table 9-1 lists and describes the most important columns of the sys.objects catalog view. The sys.columns catalog view contains a row for each column of an object that has columns, such as tables and views. Table 9-2 lists and describes the most important columns of the sys.columns catalog view. The sys.database_principals catalog view contains a row for each security principal (that is, user, group, or role in a database). (For a detailed discussion of principals, see Chapter 12.) Table 9-3 lists and describes the most important columns of the sys.objects catalog view. SQL Server 2012 still supports so-called compatibility views for backward compatibility. Each compatibility view has the same name (and the same structure) as the corresponding system base table of the SQL Server 2000 system. Compatibility views do not expose any of the metadata related to features that are introduced in SQL Server 2005 and later. They are a deprecated feature and will be removed in a future version of SQL Server. Querying Catalog Views As already stated in this chapter, all system tables have the same structure as base tables. Because system tables cannot be referenced directly, you have to query catalog views, Column Name object_id name column_id Description ID of the object to which this column belongs Column name ID of the column (unique within the object) Table 9-2 Selected Columns of the sys.columns Catalog View

6 264 Microsoft SQL Server 2012: A Beginner s Guide Column Name name principal_id type Description Name of principal ID of principal (unique within the database) Principal type Table 9-3 Selected Columns of the sys.database_principals Catalog View which correspond to particular system tables. Examples 9.1 through 9.3 use existing catalog views to demonstrate how information concerning database objects can be queried. Example 9.1 Get the table ID, user ID, and table type of the employee table: USE sample; SELECT object_id, principal_id, type FROM sys.objects WHERE name = 'employee'; The result is object_id principal_id type NULL U The object_id column of the sys.objects catalog view displays the unique ID number for the corresponding database object. The NULL value in the principal_id column indicates that the object s owner is the same as the owner of the schema. U in the type column stands for the user (table). Example 9.2 Get the names of all tables of the sample database that contain the project_no column: USE sample; SELECT sys.objects.name FROM sys.objects INNER JOIN sys.columns ON sys.objects.object_id = sys.columns.object_id WHERE sys.objects.type = 'U' AND sys.columns.name = 'project_no';

7 Chapter 9: System Catalog 265 The result is name project works_on Example 9.3 Who is the owner of the employee table? SELECT sys.database_principals.name FROM sys.database_principals INNER JOIN sys.objects ON sys.database_principals.principal_id = sys.objects.schema_id WHERE sys.objects.name = 'employee' AND sys.objects.type = 'U'; The result is name dbo Dynamic Management Views and Functions Dynamic management views (DMVs) and functions (DMFs) return server state information that can be used to observe active processes and therefore to tune system performance or to monitor the actual system state. In contrast to catalog views, the DMVs and DMFs are based on internal structures of the system. The main difference between catalog views and DMVs is in their application: catalog views display the static information about metadata, while DMVs (and DMFs) are used to access dynamic properties of the system. In other words, you use DMVs to get insightful information about the database, individual queries, or an individual user. DMVs and DMFs belong to the sys schema and their names start with the prefix dm_, followed by a text string that indicates the category to which the particular DMV or DMF belongs. The following list identifies and describes some of these categories: sys.dm_db_* Contains information about databases and their objects sys.dm_tran_* Contains information in relation to transactions

8 266 Microsoft SQL Server 2012: A Beginner s Guide sys.dm_io_* Contains information about I/O activities sys.dm_exec_* Contains information related to the execution of user code Microsoft consecutively increases the number of supported DMVs in each new version of SQL Server. SQL Server 2012 contains 20 new DMVs, so the total number is now 155. This section introduces two new DMVs: sys.dm_exec_describe_first_result_set sys.dm_db_uncontained_entities The sys.dm_exec_describe_first_result_set view describes the first result set of a group of result sets. For this reason, you can apply this DMV when several subsequent queries are declared in a batch or a stored procedure (see Example 9.4). The sys.dm_ db_uncontained_entities view shows any uncontained objects used in the database. (Uncontained objects are objects that cross the application boundary in a contained database. For the description of uncontained objects and the application boundary, see the section Contained Databases in Chapter 5.) Example 9.4 USE sample; GO CREATE PROC TwoSELECTS AS SELECT emp_no, job from works_on where emp_no BETWEEN 1000 and 9999; SELECT emp_no, emp_lname FROM employee where emp_fname LIKE 'S%'; GO SELECT is_hidden hidden,column_ordinal ord, name, is_nullable nul, system_type_id id FROM sys.dm_exec_describe_first_result_set ('TwoSELECTS', NULL, 0) ; The result is hidden ord name nul id 0 1 emp_no job 1 175

9 Chapter 9: System Catalog 267 The stored procedure in Example 9.4 contains two SELECT statements concerning the sample database. The subsequent query uses the sys.dm_exec_describe_first_ result_set view to display several properties of the result set of the first query. Many DMVs and DMFs are discussed in subsequent chapters of the book. For instance, index-related DMVs and DMFs are explained in the next chapter, while transaction-related DMVs and DMFs are discussed in Chapter 13. Information Schema The information schema consists of read-only views that provide information about all tables, views, and columns of the Database Engine to which you have access. In contrast to the system catalog that manages the metadata applied to the system as a whole, the information schema primarily manages the environment of a database. The information schema was originally introduced in the SQL92 standard. The Database Engine provides information schema views so that applications developed on other database systems can obtain its system catalog without having to use it directly. These standard views use different terminology, so when you interpret the column names, be aware that catalog is a synonym for database and domain is a synonym for user-defined data type. The following sections provide a description of the most important information schema views. Information_schema.tables The Information_schema.tables view contains one row for each table in the current database to which the user has access. The view retrieves the information from the system catalog using the sys.objects catalog view. Table 9-4 lists and describes the four columns of this view. Column TABLE_CATALOG TABLE_SCHEMA TABLE_NAME TABLE_TYPE Description The name of the catalog (database) to which the view belongs The name of the schema to which the view belongs The table name The type of the table (can be BASE TABLE or VIEW) Table 9-4 The Information_schema.tables View

10 268 Microsoft SQL Server 2012: A Beginner s Guide Column TABLE_CATALOG TABLE_SCHEMA TABLE_NAME COLUMN_NAME ORDINAL_POSITION DATA_TYPE Description The name of the catalog (database) to which the column belongs The name of the schema to which the column belongs The name of the table to which the column belongs The column name The ordinal position of the column The data type of the column Table 9-5 The Information_schema.columns View Information_schema.columns The Information_schema.columns view contains one row for each column in the current database accessible by the user. The view retrieves the information from the sys.columns and sys.objects catalog views. Table 9-5 lists and describes the six most important columns of this view. Proprietary Interfaces The previous section describes the use of the general interfaces for accessing system base tables. You can also retrieve system information using one of the following proprietary mechanisms of the Database Engine: System stored procedures System functions Property functions The following sections describe these interfaces. System Stored Procedures System stored procedures are used to provide many administrative and end-user tasks, such as renaming database objects, identifying users, and monitoring authorization and resources. Almost all existing system stored procedures access system base tables to retrieve and modify system information.

11 Chapter 9: System Catalog 269 The most important property of system stored procedures is that they can be used for easy and reliable modification of system base tables. This section describes two system stored procedures: sp_help and sp_configure. Depending on the subject matter of the chapters, certain system stored procedures were discussed in previous chapters, and additional procedures will be discussed in later chapters of the book. The sp_help system stored procedure displays information about one or more database objects. The name of any database object or data type can be used as a parameter of this procedure. If sp_help is executed without any parameter, information on all database objects of the current database will be displayed. The sp_configure system stored procedure displays or changes global configuration settings for the current server. Example 9.5 shows the use of the sp_configure system stored procedure. Example 9.5 USE sample; EXEC sp_configure 'show advanced options', 1; RECONFIGURE WITH OVERRIDE; EXEC sp_configure 'fill factor', 100; RECONFIGURE WITH OVERRIDE; Generally, you do not have access to advanced configuration options of SQL Server. For this reason, the first EXECUTE statement in Example 9.5 tells the system to allow changes of advanced options. With the next statement, RECONFIGURE WITH OVERRIDE, these changes will be installed. Now it is possible to change any of the existing advanced options. Example 9.5 changes the fill factor to 100 and installs this change. (Fill factor specifies the storage percentage for index pages and will be described in detail in the next chapter.) System Functions System functions are described in Chapter 5. Some of them can be used to access system base tables. Example 9.6 shows two SELECT statements that retrieve the same information using different interfaces.

12 270 Microsoft SQL Server 2012: A Beginner s Guide Example 9.6 USE sample; SELECT object_id FROM sys.objects WHERE name = 'employee'; SELECT object_id('employee'); The second SELECT statement in Example 9.6 uses the system function object_id to retrieve the ID of the employee table. (This information can be stored in a variable and used when calling a command, or a system stored procedure, with the object s ID as a parameter.) The following system functions, among others, access system base tables. The names of these functions are self-explanatory. OBJECT_ID(object_name) OBJECT_NAME(object_id) USER_ID([user_name]) USER_NAME([user_id]) DB_ID([db_name]) DB_NAME([db_id]) Property Functions Property functions return properties of database objects, data types, or files. Generally, property functions can return more information than system functions can return, because property functions support dozens of properties (as parameters), which you can specify explicitly. Almost all property functions return one of the following three values: 0, 1, or NULL. If the value is 0, the object does not have the specified property. If the value is 1, the object has the specified property. Similarly, the value NULL specifies that the existence of the specified property for the object is unknown to the system. The Database Engine supports, among others, the following property functions: OBJECTPROPERTY(id, property) COLUMNPROPERTY(id, column, property) FILEPROPERTY(filename, property) TYPEPROPERTY(type, property)

13 Chapter 9: System Catalog 271 The OBJECTPROPERTY function returns information about objects in the current database (see Exercise E.9.2). The COLUMNPROPERTY function returns information about a column or procedure parameter. The FILEPROPERTY function returns the specified filename and property value for a given filename and property name. The TYPEPROPERTY function returns information about a data type. (The description of existing properties for each property function can be found in Books Online.) Summary The system catalog is a collection of system base tables belonging to the master database and existing user databases. Generally, system base tables cannot be queried directly by a user. The Database Engine supports several different interfaces that you can use to access the information from the system catalog. Catalog views are the most general interface that you can apply to obtain system information. Dynamic management views (DMVs) and functions (DMFs) are similar to catalog views, but you use them to access dynamic properties of the system. System stored procedures provide easy and reliable read and write access to system base tables. It is strongly recommended to exclusively use system stored procedures for modification of system information. The information schema is a collection of views defined on system base tables that provides unified access to the system catalog for all database applications developed on other database systems. The use of the information schema is recommended if you intend to port your system from one database system to another. The next chapter introduces you to database indices. Exercises E.9.1 Using catalog views, find the operating system path and filename of the sample database. E.9.2 Using catalog views, find how many integrity constraints are defined for the employee table of the sample database. E.9.3 Using catalog views, find out if there is any integrity constraint defined for the dept_no column of the employee table.

14 272 Microsoft SQL Server 2012: A Beginner s Guide E.9.4 Using the information schema, display all user tables that belong to the AdventureWorks database. E.9.5 Using the information schema, find all columns of the employee table with their ordinal positions and the corresponding data types.