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Oracle® Database Heterogeneous Connectivity Administrator's Guide
11g Release 1 (11.1)

B28277-02
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3 Features of Oracle Database Gateways

This chapter describes the major features provided by Oracle Database Gateways. Descriptions of these features are contained in the following topics:

3.1 SQL and PL/SQL Support

SQL statements are translated and data types are mapped according to capabilities. PL/SQL calls are mapped to non-Oracle system stored procedures. In the case of SQL statements, if functionality is missing at the remote system, then either a simpler query is issued or the statement is broken up into multiple queries and the desired results are obtained by post-processing in the Oracle database.

Even though Heterogeneous Services can, for the most part, incorporate non-Oracle systems into Oracle distributed sessions, there are several limitations to this. Some of the generic limitations are:

3.2 Heterogeneous Replication

Data can be replicated between a non-Oracle system and an Oracle server using materialized views.

Note:

There is another means of replicating information between Oracle and non-Oracle databases called Streams.

For information about using Streams, see Oracle Streams Concepts and Administration.

Materialized views instantiate data captured from tables at the non-Oracle master site at a particular point in time. This instant is defined by a refresh operation, which copies this data to the Oracle server and synchronizes the copy on Oracle with the master copy on the non-Oracle system. The "materialized" data is then available as a view on the Oracle server.

Replication facilities provide mechanisms to schedule refreshes and to collect materialized views into replication groups to facilitate their administration. Refresh groups permit refreshing multiple materialized views just as if they were a single object.

Heterogeneous replication support is necessarily limited to a subset of the full Oracle-to-Oracle replication functionality:

Other restrictions apply to any access to non-Oracle data through Oracle's Heterogeneous Services facilities. The most important of these are:

The following examples illustrate basic setup and use of three materialized views to replicate data from a non-Oracle system to an Oracle data store.

Note:

For the following examples, remote_db refers to the non-Oracle system which you are accessing from your Oracle database server.

Modify these examples for your environment. Do not try to execute them as they are written.

Example 1: Create Materialized Views for Heterogeneous Replication

This example creates three materialized views that are then used in succeeding examples.

  1. Create a primary key materialized view of table customer@remote_db.

    CREATE MATERIALIZED VIEW pk_mv REFRESH COMPLETE AS
      SELECT * FROM customer@remote_db WHERE "zip" = 94555;

  1. Create a subquery materialized view of tables orders@remote_db and customer@remote_db.

    CREATE MATERIALIZED VIEW sq_mv REFRESH COMPLETE AS
      SELECT * FROM orders@remote_db o WHERE EXISTS
        (SELECT c."c_id" FROM customer@remote_db c
           WHERE c."zip" = 94555 and c."c_id"  = o."c_id" );

  1. Create a complex materialized view of data from multiple tables on remote_db.

    CREATE MATERIALIZED VIEW cx_mv
      REFRESH COMPLETE AS
      SELECT  c."c_id", o."o_id"
        FROM customer@remote_db c,
             orders@remote_db o,
             order_line@remote_db ol
        WHERE c."c_id" = o."c_id"
        AND o."o_id" = ol."o_id";

Example 2: Set Up a Refresh Group for Heterogeneous Replication

BEGIN
  dbms_refresh.make('refgroup1',
   'pk_mv, sq_mv, cx_mv',
   NULL, NULL);
 END;
 /

Example 3: Force Refresh of all Three Materialized Views

BEGIN
   dbms_refresh.refresh('refgroup1');
END;
 /

See Also:

Oracle Database Advanced Replication for a full description of materialized views and replication facilities

3.3 Passthrough SQL

The passthrough SQL feature enables you to send a statement directly to a non-Oracle system without being interpreted by the Oracle server. This feature can be useful if the non-Oracle system allows for operations in statements for which there is no equivalent in Oracle.

This section contains the following topics:

3.3.1 Using the DBMS_HS_PASSTHROUGH Package

You can execute passthrough SQL statements directly at the non-Oracle system using the PL/SQL package DBMS_HS_PASSTHROUGH. Any statement executed with this package is executed in the same transaction as standard SQL statements.

The DBMS_HS_PASSTHROUGH package is a virtual package. It conceptually resides at the non-Oracle system. In reality, however, calls to this package are intercepted by Heterogeneous Services and mapped to one or more Heterogeneous Services calls. The driver, in turn, maps these Heterogeneous Services calls to the API of the non-Oracle system. The client application should invoke the procedures in the package through a database link in exactly the same way as it would invoke a non-Oracle system stored procedure. The special processing done by Heterogeneous Services is transparent to the user.

See Also:

Oracle Database PL/SQL Packages and Types Reference for more information about this package

3.3.2 Considering the Implications of Using Passthrough SQL

When you execute a passthrough SQL statement that implicitly commits or rolls back a transaction in the non-Oracle system, the transaction is affected. For example, some systems implicitly commit the transaction containing a data definition language (DDL) statement. Because the Oracle database server is bypassed, the Oracle database server is unaware of the commit in the non-Oracle system. Consequently, the data at the non-Oracle system can be committed while the transaction in the Oracle database server is not.

If the transaction in the Oracle database server is rolled back, data inconsistencies between the Oracle database server and the non-Oracle server can occur. This situation results in global data inconsistency.

Note that if the application executes a regular COMMIT statement, the Oracle database server can coordinate the distributed transaction with the non-Oracle system. The statement executed with the passthrough facility is part of the distributed transaction.

3.3.3 Executing Passthrough SQL Statements

The following table shows the functions and procedures provided by the DBMS_HS_PASSTHROUGH package that enable you to execute passthrough SQL statements.

Procedure/Function Description
OPEN_CURSOR Opens a cursor.
CLOSE_CURSOR Closes a cursor.
PARSE Parses the statement.
BIND_VARIABLE Binds IN variables.
BIND_OUT_VARIABLE Binds OUT variables.
BIND_INOUT_VARIABLE Binds IN OUT variables.
EXECUTE_NON_QUERY Executes nonquery.
EXECUTE_IMMEDIATE Executes nonquery without bind variables.
FETCH_ROW Fetches rows from query.
GET_VALUE Retrieves column value from SELECT statement or retrieves OUT bind parameters.

3.3.3.1 Executing Nonqueries

Nonqueries include the following statements and types of statements:

  • INSERT

  • UPDATE

  • DELETE

  • DDL

To execute nonquery statements, use the EXECUTE_IMMEDIATE function. For example, to execute a DDL statement at a non-Oracle system that you can access using the database link salesdb, execute:

DECLARE
  num_rows INTEGER;

BEGIN
  num_rows := DBMS_HS_PASSTHROUGH.EXECUTE_IMMEDIATE@salesdb
            ('CREATE TABLE dept1 (n SMALLINT, loc CHARACTER(10))');
END;


The variable num_rows is assigned the number of rows affected by the execution. For DDL statements, zero is returned. Note that you cannot execute a query with EXECUTE_IMMEDIATE and you cannot use bind variables.

3.3.3.1.1 Using Bind Variables: Overview

Bind variables allow you to use the same SQL statement multiple times with different values, reducing the number of times a SQL statement needs to be parsed. For example, when you need to insert four rows in a particular table, you can parse the SQL statement once and bind and execute the SQL statement for each row. One SQL statement can have zero or more bind variables.

To execute passthrough SQL statements with bind variables, you must:

  1. Open a cursor.

  2. Parse the SQL statement at the non-Oracle system.

  3. Bind the variables.

  4. Execute the SQL statement at the non-Oracle system.

  5. Close the cursor.

Figure 3-1 shows the flow diagram for executing nonqueries with bind variables.

Figure 3-1 Flow Diagram for Nonquery Passthrough SQL

Description of Figure 3-1 follows
Description of "Figure 3-1 Flow Diagram for Nonquery Passthrough SQL"

3.3.3.1.2 Using IN Bind Variables

The syntax of the non-Oracle system determines how a statement specifies a bind variable. For example, in Oracle you define bind variables with a preceding colon. For example:

...
UPDATE emp
SET sal=sal*1.1
WHERE ename=:ename;
...

In this statement, ename is the bind variable. In non-Oracle systems, you may need to specify bind variables with a question mark. For example:

...
UPDATE emp
SET sal=sal*1.1
WHERE ename= ?;
...

In the bind variable step, you must positionally associate host program variables (in this case, PL/SQL) with each of these bind variables. For example, to execute the preceding statement, use the following PL/SQL program:


DECLARE
  c INTEGER;
  nr INTEGER;
BEGIN
  c := DBMS_HS_PASSTHROUGH.OPEN_CURSOR@salesdb;
  DBMS_HS_PASSTHROUGH.PARSE@salesdb(c,
        'UPDATE emp SET SAL=SAL*1.1 WHERE ename=?');
  DBMS_HS_PASSTHROUGH.BIND_VARIABLE@salesdb(c,1,'JONES');
  nr:=DBMS_HS_PASSTHROUGH.EXECUTE_NON_QUERY@salesdb(c);
  DBMS_OUTPUT.PUT_LINE(nr||' rows updated');
  DBMS_HS_PASSTHROUGH.CLOSE_CURSOR@salesdb(c);
END;
3.3.3.1.3 Using OUT Bind Variables

In some cases, the non-Oracle system can also support OUT bind variables. With OUT bind variables, the value of the bind variable is not known until after the execution of the SQL statement.

Although OUT bind variables are populated after the SQL statement is executed, the non-Oracle system must know that the particular bind variable is an OUT bind variable before the SQL statement is executed. You must use the BIND_OUT_VARIABLE procedure to specify that the bind variable is an OUT bind variable.

After the SQL statement is executed, you can retrieve the value of the OUT bind variable using the GET_VALUE procedure.

3.3.3.1.4 Using IN OUT Bind Variables

A bind variable can be both an IN and an OUT variable. This means that the value of the bind variable must be known before the SQL statement is executed but can be changed after the SQL statement is executed.

For IN OUT bind variables, you must use the BIND_INOUT_VARIABLE procedure to provide a value before the SQL statement is executed. After the SQL statement is executed, you must use the GET_VALUE procedure to retrieve the new value of the bind variable.

3.3.3.2 Executing Queries

The difference between queries and nonqueries is that queries retrieve a result set from a SELECT statement. The result set is retrieved by iterating over a cursor.

Figure 3-2 illustrates the steps in a passthrough SQL query. After the system parses the SELECT statement, each row of the result set can be fetched with the FETCH_ROW procedure. After the row is fetched, use the GET_VALUE procedure to retrieve the selected list of items into program variables. After all rows are fetched, you can close the cursor.

Figure 3-2 Passthrough SQL for Queries

Description of Figure 3-2 follows
Description of "Figure 3-2 Passthrough SQL for Queries"

You do not have to fetch all the rows. You can close the cursor at any time after opening the cursor.

Note:

Although you are fetching one row at a time, Heterogeneous Services optimizes the round-trips between the Oracle server and the non-Oracle system by buffering multiple rows and fetching from the non-Oracle data system in one round-trip.

The following example executes a query:

DECLARE
   val  VARCHAR2(100);
   c    INTEGER;
   nr   INTEGER;
BEGIN
  c := DBMS_HS_PASSTHROUGH.OPEN_CURSOR@salesdb; 
  DBMS_HS_PASSTHROUGH.PARSE@salesdb(c, 
    'select ENAME
     from   EMP
     where  DEPTNO=10');
  LOOP
    nr := DBMS_HS_PASSTHROUGH.FETCH_ROW@salesdb(c);
    EXIT WHEN nr = 0;
    DBMS_HS_PASSTHROUGH.GET_VALUE@salesdb(c, 1, val);
    DBMS_OUTPUT.PUT_LINE(val);
  END LOOP;  
  DBMS_HS_PASSTHROUGH.CLOSE_CURSOR@salesdb(c); 
END;

After the SELECT statement has been parsed, the rows are fetched and printed in a loop until the FETCH_ROW function returns the value 0.

3.4 Result Set Support

Various relational databases allow stored procedures to return result sets. In other words, stored procedures will be able to return one or more sets of rows.

Traditionally, database stored procedures worked exactly like procedures in any high-level programming language. They had a fixed number of arguments which could be of types IN, OUT, or IN OUT. If a procedure had n arguments, it could return at most n values as results. However, suppose that you wanted a stored procedure to execute a query such as SELECT * FROM emp and return the results. The emp table might have a fixed number of columns but there is no way of telling, at procedure creation time, the number of rows it has. Because of this, no traditional stored procedure could be created that returned the results of a such a query. As a result, several relational database vendors added the capability of returning results sets from stored procedures, but each kind of relational database returns result sets from stored procedures in a different way.

Oracle has a data type called a REF CURSOR. Like every other Oracle data type, a stored procedure can take this data type as an IN or OUT argument. In Oracle, a stored procedure must have an output argument of type REF CURSOR. It then opens a cursor for a SQL statement and places a handle to that cursor in that output parameter. The caller can then fetch from the REF CURSOR the same way as from any other cursor.

Oracle can do a lot more than simply return result sets. REF CURSOR can be passed as an input argument to PL/SQL routines to be passed back and forth between client programs and PL/SQL routines or as an input argument between several PL/SQL routines.

This section contains the following topics:

3.4.1 Result Set Support In Non-Oracle Systems

Several non-Oracle systems allow stored procedures to return result sets but do so in completely different ways. Result sets are supported to some extent in DB2, Sybase, Microsoft SQL Server, and Informix. Result set support in these databases is based on one of the following two models.

3.4.1.1 Model 1: Result Set Support

When creating a stored procedure, you can explicitly specify the maximum number of result sets that can be returned by that stored procedure. While executing, the stored procedure can open anywhere from zero up to its prespecified maximum number of result sets. After the execution of the stored procedure, a client program obtains handles to these result sets by using either an embedded SQL directive or by calling a client library function. After that, the client program can fetch from the result set in the same way as from a regular cursor.

3.4.1.2 Model 2: Result Set Support

In this model, there is no prespecified limit to the number of result sets that can be returned by a stored procedure. Both Model 1 and Oracle have a limit. For Oracle, the number of result sets returned by a stored procedure can be at most the number of REF CURSOR OUT arguments. For Model 1, the upper limit is specified using a directive in the stored procedure language. Another way that Model 2 differs from Oracle and Model 1 is that they do not return a handle to the result sets but instead place the entire result set on the wire when returning from a stored procedure. For Oracle, the handle is the REF CURSOR OUT argument. For Model 1, it is obtained separately after the execution of the stored procedure. For both Oracle and Model 1, once the handle is obtained, data from the result set is obtained by doing a fetch on the handle; there are several cursors open and the fetch can be in any order. In the case of Model 2, however, all the data is already on the wire, with the result sets coming in the order determined by the stored procedure and the output arguments of the procedures coming at the end. So the whole of the first result set must be fetched, then the whole of the second one, until all of the results have been fetched. Finally, the stored procedure OUT arguments must be fetched.

3.4.2 Heterogeneous Services Support for Result Sets

As can be seen in the preceding sections, result set support exists among non-Oracle databases in a variety of forms. All of these have to be mapped onto the Oracle REF CURSOR model. Due to the considerable differences in behavior among the various non-Oracle systems, Heterogeneous Services result set support behaves in one of two different ways depending on the non-Oracle system to which it is connected.

Please note the following about Heterogeneous Services result set support:

  • Result set support is present in Heterogeneous Services generic code but in order for the feature to work in a gateway, the driver has to implement it as well. Not all drivers have implemented result set support and you should check in the gateway-specific documentation to determine whether it is supported in your gateway.

  • Heterogeneous Services supports REF CURSOR OUT arguments from stored procedures. IN and IN OUT arguments are not supported.

  • The REF CURSOR OUT arguments are all anonymous REF CURSORs. No typed REF CURSORs are returned by Heterogeneous Services.

3.4.2.1 Results Sets: Cursor Mode

Each result set returned by the non-Oracle system stored procedure is mapped by the Oracle driver to an OUT argument of type REF CURSOR. The client program sees a stored procedure with several OUT arguments of type REF CURSOR. After executing the stored procedure, the client program can fetch from the REF CURSOR in the same way as it would from a REF CURSOR returned by an Oracle stored procedure. When connecting to the gateway as described in Section 3.4.1.1, Heterogeneous Services will be in cursor mode.

3.4.2.2 Result Sets: Sequential Mode

In Oracle, there is a prespecified maximum number of result sets that a particular stored procedure can return. The number of result sets returned is at most the number of REF CURSOR OUT arguments for the stored procedure. It can return fewer result sets, but it can never return more.

For the system described in Section 3.4.1.2, there is no prespecified maximum number of result sets that can be returned. In the case of Model 1 (in Section 3.4.1.1), we know the maximum number of result sets that a procedure can return, and that the driver can return to Heterogeneous Services, is prespecified in the stored procedure by the number of REF CURSOR OUT arguments. If, on execution of the stored procedure, fewer result sets than the maximum are returned, then the other REF CURSOR OUT arguments are set to NULL.

Another problem for Model 2 database servers is that result sets have to be retrieved in the order in which they were placed on the wire by the database. This prevents Heterogeneous Services from running in cursor mode when connecting to these databases. To access result sets returned by these stored procedures, you must operate Heterogeneous Services in sequential mode.

In sequential mode, the procedure description returned by the driver contains the following:

  • All the input arguments of the remote stored procedure

  • None of the output arguments

  • One OUT argument of type REF CURSOR (corresponding to the first result set returned by the stored procedure)

The client fetches from this REF CURSOR and then calls the virtual package function DBMS_HS_RESULT_SET.GET_NEXT_RESULT_SET to fetch the REF CURSOR corresponding to the next result set. This function call is repeated until all result sets have been fetched. The last result set returned will actually be the OUT arguments of the remote stored procedure.

The major limitations of sequential mode are as follows:

  • Result sets returned by a remote stored procedure have to be retrieved in the order in which they were placed on the wire.

  • On execution of a stored procedure, all result sets returned by a previously executed stored procedure are closed (regardless of whether the data has been completely fetched or not).

    See Also:

    Your gateway-specific manual for more information about how result sets are supported through the gateway

3.5 Data Dictionary Translations

Most database systems have some form of data dictionary. A data dictionary is a collection of information about the database objects that have been created by various users of the system. For a relational database, a data dictionary is a set of tables and views which contain information about the data in the database. This information includes information on the users who are using the system and on the objects that they have created (such as tables, views, and triggers). For the most part, all data dictionaries (regardless of the database system) contain the same information but each database system organizes the information in a different way.

For example, the ALL_CATALOG Oracle data dictionary view gives a list of tables, views, and sequences in the database. It has three columns: the first is called OWNER and is the name of the owner of the object, the second is called TABLE_NAME and is the name of the object, and the third is called TABLE_TYPE and is the type. This field has value TABLE, VIEW, SEQUENCE and so forth depending on the object type. However, in Sybase, the same information is stored in two tables called sysusers and sysobjects whose column names are quite different from those of the Oracle ALL_CATALOG table. Additionally, in Oracle, the table type is a string with a value such as TABLE or VIEW. In Sybase, it is a letter. For example, in Sybase, U means user table, S means system table, V means view, and so forth.

If the client program wants information from the table ALL_CATALOG at Sybase, it sends a query referencing ALL_CATALOG@database_link to a gateway. Heterogeneous Services translates this query to an appropriate query on systables and then sends the translated query to Sybase. For example:

SELECT SU."name" OWNER, SO."name" TABLE_NAME,
   DECODE(SO."type", 'U ','TABLE', 'S ', 'TABLE', 'V ', 'VIEW')
TABLE_TYPE
FROM "dbo"."sysusers"@remote_db  SU, "dbo"."sysobjects"@remote_db SO
WHERE SU."uid" = SO."uid" AND
   (SO."type" = 'V' OR SO."type" = 'S' OR SO."type" = 'U');

To relay the translation of a query on an Oracle data dictionary table to the equivalent one on the non-Oracle system data dictionary table, Heterogeneous Services needs data dictionary translations for that non-Oracle system. A data dictionary translation is a view definition (essentially a SELECT statement) of one or more non-Oracle system data dictionary tables that look exactly like the Oracle data dictionary table, with the same column names and the same information formatting. A data dictionary translation may not be as simple as the preceding example. Often the information needed is scattered over many tables and the data dictionary translation is a complex join of those tables.

In some cases, an Oracle data dictionary table does not have a translation because the information needed does not exist at the non-Oracle system. In such cases, the gateway can decide not to upload a translation at all or can resort to an alternative approach called mimicking. If the gateway wants to mimic a data dictionary table, it will let Heterogeneous Services know and Heterogeneous Services will obtain the description of the data dictionary table by querying the local database but when asked to fetch data, it will report that no rows were selected.

3.6 Datetime Data Types

Oracle has five datetime data types:

Heterogeneous Services generic code supports Oracle datetime data types in SQL and stored procedures. Heterogeneous Services do not support these data types in data dictionary translations or queries involving data dictionary translations.

Even though Heterogeneous Services generic code supports this, support for a particular gateway depends on whether or not the driver for that non-Oracle system has implemented datetime support. Support, even when the driver implements it, may be partial because of the limitations of the non-Oracle system. You should consult the documentation for your particular gateway on this issue.

You must set the timestamp formats of the non-Oracle system in the gateway initialization file. The parameters to set are HS_NLS_TIMESTAMP_FORMAT and HS_NLS_TIMESTAMP_TZ_FORMAT. You should also set the local time zone for the non-Oracle system in the initialization file by setting HS_TIME_ZONE.

See Also:

Oracle Database SQL Language Reference for information about datetime data types

3.7 Two-Phase Commit Protocol

Heterogeneous Services provides the infrastructure for the implementation of the two-phase commit mechanism. The extent to which this is supported depends on the gateway and the remote system. Refer to individual gateway manuals for more information.

See Also:

Oracle Database Administrator's Guide for more information about the two-phase commit protocol

3.8 Piecewise LONG Data Type

Earlier versions of gateways had limited support for the LONG data type. LONG is an Oracle data type that can be used to store up to 2 gigabytes (GB) of character data or raw data (LONG RAW). These earlier versions restricted the amount of LONG data to 4 MB because they treated LONG data as a single piece. This led to restrictions of memory and network bandwidth on the size of the data that could be handled. Current gateways have extended the functionality to support the full 2 GB of heterogeneous LONG data. They handle the data piecewise between the agent and the Oracle server, thereby doing away with the large memory and network bandwidth requirements.

The HS_LONG_PIECE_TRANSFER_SIZE Heterogeneous Services initialization parameter can be used to set the size of the transferred pieces. For example, consider fetching 2 GB of LONG data from a heterogeneous source. A smaller piece size means less memory requirement, but more round-trips to fetch all the data. A larger piece size means fewer round-trips, but more of a memory requirement to store the intermediate pieces internally. Thus the initialization parameter can be used to tune a system for the best performance, that is, for the best trade off between round-trips and memory requirements. If the initialization parameter is not set, the system defaults to a piece size of 64 KB.

Note:

Do not confuse this feature with piecewise operations on LONG data on the client side. Piecewise fetch and insert operations on the client side did work with the earlier versions of the gateways, and continue to do so. The only difference on the client side is that, where earlier versions of the gateways were able to fetch a maximum of 4 megabytes (MB) of LONG data, now they can fetch the entire 2 GB of LONG data. This is a significant improvement, considering that 4 MB is only 0.2% of the data type's full capacity.

3.9 SQL*Plus DESCRIBE Command

You can describe non-Oracle system objects using the SQL*Plus DESCRIBE command. However, there are some limitations. For example, using heterogeneous links, you cannot describe packages, sequences, synonyms, or types.

3.10 Constraints on SQL in a Distributed Environment

This section explains some of the constraints on SQL in a distributed environment. These constraints apply to distributed environments that access non-Oracle systems or remote Oracle databases.

This section contains the following topics:

3.10.1 Remote and Heterogeneous References

Note:

Many of the rules for heterogeneous access also apply to remote references. For more information, please see the distributed database section of the Oracle Database Administrator's Guide.

A statement can, with restrictions, be executed on any database node referenced in the statement or the local node. If all objects referenced are resolved to a single, referenced node, Oracle attempts to execute a query at that node. You can force execution at a referenced node by using the /*+ REMOTE_MAPPED */ or /*+ DRIVING_SITE */ hints. If a statement is forwarded to a node other than the node from where the statement was issued, the statement is said to be remote-mapped.

There is complete data type checking support for remote-mapped statements. The result provides consistent data type checking and complete data type coercion.

See Also:

Section 4.5, "Oracle Database Server SQL Construct Processing"

The ways in which statements can be remote-mapped are subject to specific rules or restrictions. If these rules are not followed, an error occurs. As long as the statements issued are consistent with all these rules, the order in which the rules are applied does not matter. See Section 3.10.2 for these rules or restrictions.

Different constraints exist when you are using SQL for remote mapping in a distributed environment. This distributed environment can include remote Oracle databases as well as non-Oracle databases that are accessed through Oracle Database Gateways.

3.10.2 Important Restrictions

The following section lists some of the different rules or restrictions that exist when you are using SQL for remote mapping in a distributed environment.

Note:

In the examples that follow, remote_db refers to a remote non-Oracle system while remote_oracle_db refers to a remote Oracle server.

Rule A: A data definition language statement cannot be remote-mapped.

In Oracle data definition language, the target object syntactically has no place for a remote reference. Data definition language statements that contain remote references are always executed locally. For Heterogeneous Services, this means it cannot directly create database objects in a non-Oracle database using SQL.

However, there is an indirect way using passthrough SQL as shown in the following example:

DECLARE
  num_rows INTEGER;
BEGIN 
  num_rows := DBMS_HS_PASSTHROUGH.EXECUTE_IMMEDIATE@remote_db 
  (
     'create table x1 (c1 char, c2 int)'
  ); 
END; 

Rule B: INSERT, UPDATE and DELETE statements with a remote target table must be remote-mapped.

This rule is more restrictive for non-Oracle remote databases than for a remote Oracle database. This is because the remote system cannot fetch data from the originating Oracle database while executing data manipulation language (DML) statements targeting tables in a non-Oracle system.

For example, to insert all local employees from the local emp table to a remote non-Oracle emp table, use the following statement:

This statement is remote-mapped to the remote database. The remote-mapped statement sent to the remote database contains a remote reference back to the originating database for emp. Such a remote link received by the remote database is called a callback link.

Note:

Even though callback links are supported in generic Heterogeneous Services, they may not be implemented in all Heterogeneous Services agents. Refer to your database gateway documentation to determine if callback links work with the database gateway that you are using.

If callback links are not supported by a particular gateway, the previous INSERT statements returns the following error:

ORA-02025: all tables in the SQL statement must be at the remote database

The workaround is to write a PL/SQL block. For example:

DECLARE 
CURSOR remote_insert IS SELECT * FROM emp;
BEGIN 
   FOR rec IN remote_insert LOOP
    INSERT INTO emp@remote_db (empno, ename, deptno) VALUES (
      rec.empno,
      rec.ename, 
      rec.deptno 
    );  
   END loop;
END;

Another special case involves session-specific SQL functions such as USER, USERENV, and SYSDATE. These functions need to be executed at the originating site. A remote-mapped statement containing these functions contains a callback link. For a non-Oracle database for which callbacks are not supported, this can (by default) result in a restriction error.

For example, consider the following statement:

DELETE FROM emp@remote_db WHERE hiredate > sysdate;

The previous statement returns the following error message:

ORA-02070: database REMOTE_DB does not support special functions in this context 

This can be resolved by replacing special functions with a bind variable. For example:

DELETE FROM emp@remote_db WHERE hiredate > :1;

Rule C: Object features like tables with nested table columns, ADT columns, Opaque columns, or Ref Columns cannot be remote-mapped.

Currently, these column types are not supported for heterogeneous access. Hence, this limitation is not directly encountered.

Rule D: SQL statements containing operators and constructs that are not supported at the remote site cannot be remote-mapped.

In the case of an INSERT, UPDATE, or DELETE, this means that the SQL statement cannot be executed (see Rule B). However, you might still be able to execute the SQL statement if the unsupported operator or construct can be executed through a callback link.

In the case of a SELECT, you can always execute a statement affected by this rule as long as none of the remaining rules require the statement to be remote mapped. The SELECT statements affected by this rule are executed by fetching all the necessary data through a remote SELECT operation, and processing the unsupported operator or construct locally using the local SQL engine.

A remote SELECT operation is the operation that retrieves rows from the remote table as opposed to an operation that retrieves data from the local table. In the worse possible case of a remote SELECT, Oracle does a full table scan. A full table scan is a fetch of all the data in the remote table across the network without any filtering (for example, SELECT * FROM EMP).

Full table scans are expensive and, therefore, Oracle attempts to avoid them. If there are indexes on the remote table that can be used, these indexes are used in a WHERE clause predicate to reduce the number of rows fetched across the network.

You can check the SQL statement generated by the Oracle server by explaining the statement and querying the OTHER column of the explain plan table for each REMOTE operation.

See Also:

Section 3.11.1 for more information on how to interpret explain plans with remote references

For example, consider the following statement:

SELECT COUNT(*) FROM emp@remote_db WHERE hiredate < sysdate;

The statement returns the following output:

COUNT(*)  
----------
        14
1 row selected.

The remote table scan is:

SELECT hiredate FROM emp;

The predicate converted to a filter cannot be generated back and passed down to the remote operation because sysdate is not supported by the remote_db or evaluation rules. Thus sysdate must be executed locally.

Note:

Because the remote table scan operation is only partially related to the original query, the number of rows retrieved can be significantly larger than expected and can have a significant impact on performance.

Rule E: SQL statement containing a table expression cannot be remote-mapped.

This limitation is not directly encountered because table expressions are not supported in the heterogeneous access module.

Rule F: If a SQL statement selects LONG data, the statement must be mapped to the node where the table containing the LONG data resides.

Consider the following type of statement:

SELECT long1 FROM table_with_long@remote_db, dual; 

The previous statement returns the following error message (if callback links are not supported):

ORA-02025: all tables in the SQL statement must be at the remote database

Rule G: The statement must be mapped to the node on which the table or tables with columns referenced in the FOR UPDATE OF clause resides when the SQL statement is of form "SELECT...FOR UPDATE OF..."

When the SQL statement is of the form SELECT...FOR UPDATE OF..., the statement must be mapped to the node on which the table or tables with columns referenced in the FOR UPDATE OF clause resides.

For example, consider the following statement:

SELECT ename FROM emp@remote_db WHERE hiredate < sysdate FOR UPDATE OF empno;

The previous statement returns the following error message if it cannot be remote-mapped:

ORA-02070: database REMOTE_DB does not support special functions in this context 

Rule H: If the SQL statement contains a SEQUENCE or sequences, then the statement must be mapped to the site where each sequence resides.

This rule is not encountered for the heterogeneous access module because remote non-Oracle sequences are not supported.

Rule I: If the statement contains a user-defined operator or operators, then the statement must be mapped to the node where each operator is defined.

If the statement contains a user-defined operator, the entire statement needs to be remote-mapped to the database node where the operator is defined.

Rule J: A statement containing duplicate bind variables cannot be remote-mapped.

The workaround for this restriction is to use unique bind variables and bind by number.

3.11 Using Oracle's Optimizer with Heterogeneous Services

Oracle's optimizer can be used with Heterogeneous Services. Heterogeneous Services collects certain table and index statistics information on the respective non-Oracle system tables and passes this information back to the Oracle server. The Oracle cost-based optimizer uses this information when building the query plan.

There are several other optimizations that the cost-based optimizer performs. The most important ones are remote sort elimination and remote joins.

This section contains the following topics:

3.11.1 Example: Using Index and Table Statistics

Consider the following statement where you create a table in the Oracle database with 10 rows:

CREATE TABLE T1 (C1 number);

Analyze the table using the DBMS_STATS package. For example:

DBMS_STATS.GATHER_TABLE_STATS ('SCOTT','T1');
DBMS_STATS.GENERATE_STATS ('SCOTT','T1');

The preceding example assumes the schema name is SCOTT and the table name is T1. See the Oracle Database PL/SQL Packages and Types Reference for more information about the DBMS_STATS package.

Create a table in the non-Oracle system with 1000 rows.

Issue the following SQL statement:

SELECT a.* FROM remote_t1@remote_db a, T1 b 
    WHERE a.C1 = b.C1;

The Oracle optimizer issues the following SQL statement to the agent:

SELECT C1 FROM remote_t1@remote_db;

This fetches all 1000 rows from the non-Oracle system and performs the join in the Oracle database.

If we add a unique index on the column C1 in the table remote_t1, and issue the same SQL statement again, the agent receives the following SQL statement for each value of C1 in the local t1:

...
SELECT C1 FROM remote_t1@remote_db WHERE C1 = ?;
...

Note:

? is the bind parameter marker. Also, join predicates containing bind variables generated by Oracle are generated only for nested loop join methods.

To verify the SQL execution plan, generate an explain plan for the SQL statement. First, load utlxplan in the admin directory.

Enter the following:

EXPLAIN PLAN FOR SELECT a.* FROM remote_t1@remote_db a, T1 b 
    WHERE a.C1 = b.C1;

Execute the utlxpls utility script by entering the following statement.

@utlxpls

OPERATION REMOTE indicates that remote SQL is being referenced.

To find out what statement is sent, enter the following statement:

SELECT ID, OTHER FROM PLAN_TABLE WHERE OPERATION = 'REMOTE';

3.11.2 Example: Remote Join Optimization

The following is an example of the remote join optimization capability of the Oracle database.

Note:

The explain plan that uses tables from a non-Oracle system can differ from similar statements with local or remote Oracle table scans. This is because of the limitation on the statistics available to Oracle for non-Oracle tables. Most importantly, column selectivity is not available for non-unique indexes of non-Oracle tables. Because of the limitation of the statistics available, the following example is not necessarily what you encounter when doing remote joins and is intended for illustration only.

Consider the following example:

EXPLAIN PLAN FOR
SELECT e.ename, d.dname, f.ename, f.deptno FROM
   dept d,
   emp@remote_db e,
   emp@remote_db f
 WHERE e.mgr = f.empno
  AND e.deptno = d.deptno 
  AND e.empno = f.empno;
  
@utlxpls

You should see output similar to the following:

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

---------------------------------------------------------------------------
| Id    | Operation                           | Name  | Rows  | Bytes  | Cost
| Inst  |IN-OUT|
---------------------------------------------------------------------------
|   0   | SELECT STATEMENT   |                |  2000   |   197K|   205 |
|*  1   | HASH JOIN          |                |  2000   |   197K|
205 |
|   2   | TABLE ACCESS FULL  | DEPT           |  21     |   462 |     2 |
|*  3   | HASH JOIN          |                |  2000   |   154K|
201 |
|   4   | REMOTE             |                |  2000   | 66000 
|    52 |
|   5   | REMOTE             |                |  2000   | 92000
|    52 |
---------------------------------------------------------------------------

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

Query Block Name / Hint Alias (identified by operation id):
-----------------------------------------------------------

   1 - sel$1 / D
   2 - sel$1 / D
   3 - sel$1 / F
   4 - sel$1 / F
   5 - sel$1 / E

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

---------------------------------------------------

   1 - access("E"."DEPTNO"="D"."DEPTNO")
   3 - access("E"."MGR"="F"."EMPNO" AND "E"."EMPNO"="F"."EMPNO")


Issue the following statement:

SET long 300
SELECT other FROM plan_table WHERE operation = 'REMOTE'; 

You should see output similar to the following:

OTHER
--------------------------------------------------------------------------------

SELECT "EMPNO","ENAME","DEPTNO" FROM "EMP"
SELECT "EMPNO","ENAME","MGR","DEPTNO" FROM "EMP"
SELECT "EMPNO","ENAME","DEPTNO" FROM "EMP"
SELECT "EMPNO","ENAME","MGR","DEPTNO" FROM "EMP"

3.11.3 Optimizer Restrictions for Non-Oracle Access

The following are optimizer restrictions for non-Oracle system access:

  • There are no column statistics for remote objects. This can result in poor execution plans. Verify the execution plan and use hints to improve the plan.

  • There is no optimizer hint to force a remote join. However, there is a remote query block optimization that can be used to rewrite the query slightly in order to get a remote join.

    The example from the previous section can be rewritten to the following form:

        SELECT v.ename, d.dname, d.deptno FROM dept d,
            (SELECT /*+ NO_MERGE */ 
             e.deptno deptno, e.ename ename emp@remote_db e, emp@remote_db f
                 WHERE e.mgr = f.empno
                 AND e.empno = f.empno;
            )
          WHERE v.deptno = d.deptno;
    
    
    

This example guarantees a remote join because it has been isolated in a nested query with the NO_MERGE hint.