Usually you have to dig through SQL references to find out how to get metadata information for specific DBMS. There are few servers that support same SQL commands. Sometimes the syntax differs slightly, sometimes a server does not support certain statement. Now you can forget about those problems because dotConnect for PostgreSQL retrieves the metadata for you.
Since dotConnect for PostgreSQL 2.00 you can take advantage of a very useful feature - GetSchema method. It allows you to read server schema information without writing queries and parsing the output. All information you may want to obtain is brought to you by single function in easy-to-process format. You can get information on databases, tables, columns, indexes, users, stored procedures and functions, user-defined procedures, and reserved words. The method is introduced in System.Data.Common.DbConnection.
GetSchema method is available in three overloads, each of them serves its own purpose. All overloads return System.Data.DataTable object that contains information about server elements.
If you call the GetSchema method without parameters, or with single parameter "MetaDataCollections" (which is actually the same), the table object returned by the method will contain three columns. The first field of every row is a keyword allowed to be passed to the method (as collectionName argument). The second field is the number of restriction values for this keywords (passed through restrictionValues argument). The third field is not used in dotConnect for PostgreSQL. It is always zero.
GetSchema with 1 argument returns general information about the collection queried. For example, GetSchema("Users") returns list of users on the server.
In this overload first parameter is name of a collection, and second parameter is the array of restrictions to be applied when querying information. These restrictions specify which subset of the collection will be returned. The restrictions can include, for example, the database name (in this case, only collection elements belonging to this database will be returned) or the mask for the name of collection elements (only the elements satisfying this mask will be returned). The quantity and description of restrictions allowed for each metadata collection are represented in the table here. Their number can also be obtained from the return of the GetSchema() method. If the second parameter is null/Nothing, it is ignored.
Instead of specifying the metadata collection name as a string constant, you may use members of System.Data.DbMetaDataCollectionNames and Devart.Data.PostgreSql.PgSqlMetadataCollectionNames as the first GetSchema argument values. The members of these classes are the string fields, each field stores the corresponding metadata collection name. It is recommended to use these fields rather than manually input the collection names manually as the string constants because in case of using these fields, you will find misspellings at compile-time, and intellisense will show you all the available metadata collection names.
GetSchema supports two kinds of wildcards: "%" and "_". These special characters behave exactly as in LIKE operator. "%" means zero or more of any characters, "_" means any single character.
When calling the GetSchema method, you can pass all or few of restrictions, applicable to the chosen collection. In the latter case, you have to put "%" wildcard in the place of omitted restriction, if it must be specified before the restriction you want to apply. For example, GetSchema("Casts", new string[] { "%", "%", "date" }) returns list of all casts to type "date". Or, suppose you want to query all functions within a certain schema: GetSchema("Functions", new string[] { "%", "public" }) returns list of all functions in schema public. Note that this collection may accept 8 restrictions, however, you do not need to specify all of them.
The following table provides detailed information on metadata collections that can be retrieved using the GetSchema method, and restrictions that can be applied for them. Some collections may be not supported in older server versions. If you try to get metadata for unsupported collection you will get exception with message "Collection not defined".
Collection Name |
Number of restrictions | Remarks |
---|---|---|
MetaDataCollections | 0 |
Returns this list. Same as using GetSchema() method without parameters. |
ReservedWords | 0 |
Lists all reserved words used in the server. |
AggregateFunctions | 2 |
GetSchema("AggregateFunctions") returns detailed list of all aggregate functions on the server.
|
Arguments | 5 |
GetSchema("Arguments") returns the list of the function/procedure arguments. The following restrictions may be specified:
|
Casts | 3 |
GetSchema("Casts") returns the list of casts available on the server.
|
CompositeTypeAttributes | 4 |
GetSchema("CompositeTypeAttributes") returns the list of complex type attributes available on the server.
|
Columns | 3 |
Returns list of columns, their schema, table, type, and some extra information.
|
Databases | 3 |
Returns list of databases on the server.
|
DatasourceInformation | 1 |
Returns information about data sources. The only restriction is user name. |
Domains | 9 |
Returns list of domains on the server. The restrictions for this collection are the following:
|
ForeignKeys | 3 |
Returns list of foreign key columns on the server. The restrictions for this collection are the following:
|
ForeignKeyColumns | 4 |
Returns list of foreign keys on the server. The restrictions for this collection are the following:
|
Functions | 8 |
Returns list of functions on the server. The following restrictions may be specified:
|
Groups | 1 |
Lists all groups on the server, including IDs and members. The only restriction is a group name. |
Indexes | 4 |
Returns list of indexes and their details. Restricted by:
|
IndexColumns | 3 |
Returns list of index columns and their details. Restricted by:
|
Languages | 2 |
Lists all languages on the server. The first restriction is a language name, the second is OID. |
NestedTables | 6 |
Lists table inheritance relationships on the server. Restricted by:
|
Operators | 2 |
Returns list of operators allowed on the server. Restrict it by a name (the first restriction) and OID (the second restriction). |
OperatorClasses | 6 |
Lists operator classes on the server. Restricted by:
|
PrimaryKeys | 4 |
Returns list of primary keys on the server.
|
Procedures | 6 |
Returns list of functions on the server. The following restrictions may be specified:
|
Rules | 4 |
Returns list of rules on the server.
|
Schemas | 2 |
Returns list of schemas on the server.
|
Sequences | 4 |
Returns list of sequences on the server.
|
Tables | 3 |
Returns list of tables on the server. Restrict the collection with:
|
Triggers | 6 |
Returns list of trigger functions on the server. Restricted by:
|
Types | 5 |
Returns list of types on the server. Restricted by:
|
UniqueKeys | 4 |
Returns list of unique keys on the server.
|
UserPrivileges | 1 |
Returns information about privileges of users on the server. When restricted by username, returns information about specific user. |
Users | 1 |
Lists all users on the server. When restricted by username, returns information about specific user. |
Views | 3 |
Returns list of types on the server. Restricted by:
|
The following code fragment is an elegant way to detect existence of a table.
The next sample shows how to retrieve columns information from a table and render it to console.
The following sample demonstrates how to generate SQL CREATE TABLE statement basing on metadata retrieved with GetSchema method. The generated script will work with all database management systems that support ANSI standard. Only column name and type are included in the script.
Also you can get a metadata of query result set using the GetSchemaTable method of PgSqlDataReader and the ShemaTable property of PgSqlDataTable classes.