7
Performance Guidelines

This chapter discusses the following topics:

• LOB Performance Guidelines
• Moving Data to LOBs in a Threaded Environment

LOB Performance Guidelines

This section describes performance guidelines for applications that use LOB datatypes.

Performance Guidelines for Small Size LOBs

If most LOBs in your database tables are small in size--8K bytes or less--and only a few rows have LOBs larger than 8K bytes, then use the following guidelines to maximize database performance:

• Use ENABLE STORAGE IN ROW
• Set the DB_BLOCK_SIZE initialization parameter to 8K bytes and use a chunk size of 8K bytes
• See "LOB Storage" information on tuning other parameters such as CACHE, PCTVERSION, and INITIAL and NEXT for the LOB segment.

General Performance Guidelines

Use the following guidelines to achieve maximum performance with LOBs:

• When Possible, Read/Write Large Data Chunks at a Time: Because LOBs are big, you can obtain the best performance by reading and writing large chunks of a LOB value at a time. This helps in several respects:
  1. If accessing the LOB from the client side and the client is at a different node than the server, then large reads/writes reduce network overhead.
  2. If using the 'NOCACHE' option, then each small read/write incurs an I/O. Reading/writing large quantities of data reduces the I/O.
  3. Writing to the LOB creates a new version of the LOB chunk. Therefore, writing small amounts at a time will incur the cost of a new version for each small write. If logging is on, then the chunk is also stored in the redo log.
• Use LOB Buffering to Read/Write Small Chunks of Data: If you need to read/write small pieces of LOB data on the client, then use LOB buffering -- see OCILobEnableBuffering(), OCILobDisableBuffering(), OCILobFlushBuffer(), OCILobWrite2(), OCILobRead2(). Basically, turn on LOB buffering before reading/writing small pieces of LOB data.

  See Also: "LOB Buffering Subsystem" for more information on LOB buffering.

• Use OCILobRead2() and OCILobWrite2() with Callback: So that data is streamed to and from the LOB. Ensure the length of the entire write is set in the 'amount' parameter on input. Whenever possible, read and write in multiples of the LOB chunk size.
• Use a Checkout/Checkin Model for LOBs: LOBs are optimized for the following operations:
  • SQL UPDATE which replaces the entire LOB value
  • Copy the entire LOB data to the client, modify the LOB data on the client side, copy the entire LOB data back to the database. This can be done using OCILobRead2() and OCILobWrite2() with streaming.
• Commit changes frequently.

Temporary LOB Performance Guidelines

In addition to the guidelines described earlier under "LOB Performance Guidelines" on LOB performance in general, here are some guidelines for using temporary LOBs:

• Use a separate temporary tablespace for temporary LOB storage instead of the default system tablespace. This avoids device contention when copying data from persistent LOBs to temporary LOBs.

  If you use the newly provided enhanced SQL semantics functionality in your applications, then there will be many more temporary LOBs created silently in SQL and PL/SQL than before. Ensure that temporary tablespace for storing these temporary LOBs islarge enough for your applications. In particular, these temporary LOBs are silently created when you use the following:

  • SQL functions on LOBs
  • PL/SQL built-in character functions on LOBs
  • Variable assignments from VARCHAR2/RAW to CLOBs/BLOBs, respectively.
  • Perform a LONG-to-LOB migration
• In PLSQL, use NOCOPY to pass temporary LOB parameters by reference whenever possible. Refer to the PL/SQL User's Guide and Reference, for more information on passing parameters by reference and parameter aliasing.
• Take advantage of buffer cache on temporary LOBs. Temporary LOBs created with the CACHE parameter set to true move through the buffer cache. Otherwise temporary LOBs are read directly from, and written directly to, disk.
• For optimal performance, temporary LOBs use reference on read, copy on write semantics. When a temporary LOB locator is assigned to another locator, the physical LOB data is not copied. Subsequent READ operations using either of the LOB locators refer to the same physical LOB data. On the first WRITE operation after the assignment, the physical LOB data is copied in order to preserve LOB value semantics, that is, to ensure that each locator points to a unique LOB value. This performance consideration mainly applies to the PL/SQL and OCI environments.

  In PL/SQL, reference on read, copy on write semantics are illustrated as follows:

  
  LOCATOR1 BLOB; 
  LOCATOR2 BLOB; 
  DBMS_LOB.CREATETEMPORARY (LOCATOR1,TRUE,DBMS_LOB.SESSION); 
  
  -- LOB data is not copied in this assignment operation:  
  LOCATOR2 := LOCATOR;  
  -- These read operations refer to the same physical LOB copy: 
  DBMS_LOB.READ(LOCATOR1, ...); 
  DBMS_LOB.GETLENGTH(LOCATOR2, ...); 
  
  -- A physical copy of the LOB data is made on WRITE:  
  DBMS_LOB.WRITE(LOCATOR2, ...); 
  
  

  In OCI, to ensure value semantics of LOB locators and data, OCILobLocatorAssign() is used to copy temporary LOB locators as well as the LOB Data. OCILobLocatorAssign() does not make a round trip to the server. The physical temporary LOB copy is made when LOB updates happen in the same round trip as the LOB update API as illustrated in the following:

  
  OCILobLocator *LOC1;
  OCILobLocator *LOC2;
  OCILobCreateTemporary(... LOC1, ... TRUE,OCI_DURATION_SESSION);
  
  /* No round-trip is incurred in the following call. */
  OCILobLocatorAssign(... LOC1, LOC2);
  
  /* Read operations refer to the same physical LOB copy. */
  OCILobRead2(... LOC1 ...)
  
  /* One round-trip is incurred to make a new copy of the
   * LOB data and to write to the new LOB copy.
   */
  OCILobWrite2(... LOC1 ...)
  
  /* LOC2 does not see the same LOB data as LOC1. */
  OCILobRead2(... LOC2 ...)
  
  
  

  If LOB value semantics are not intended, then you can use C pointers to achieve reference semantics as illustrated in the following:

  OCILobLocator *LOC1;
  OCILobLocator *LOC2;
  OCILobCreateTemporary(... LOC1, ... TRUE,OCI_DURATION_SESSION);
  
  /* Pointer is copied. LOC1 and LOC2 refer to the same LOB data. */
  LOC2 = LOC1;
  
  /* Write to LOC2. */
  OCILobWrite2(...LOC2...)
  
  /* LOC1 will see the change made to LOC2. */
  OCILobRead2(...LOC1...)
  
  

• Use OCI_OBJECT mode for temporary LOBs

  To improve the performance of temporary LOBs on LOB assignment, use OCI_OBJECT mode for OCILobLocatorAssign. In OCI_OBJECT mode, the database tries to minimize the number of deep copies to be done. Hence, after OCILobLocatorAssign is done on a source temporary LOB in OCI_OBJECT mode, the source and the destination locators will point to the same LOB until any modification is made through either LOB locator.

• Free up temporary LOBs returned from SQL queries and PLSQL programs.

  In PL/SQL, C (OCI), Java and other programmatic interfaces, SQL query results or PLSQL program executions return temporary LOBs for operation/function calls on LOBs. For example:

    SELECT substr(CLOB_Column, 4001, 32000) FROM ... 
  
  

  If the query is executed in PLSQL, then the returned temporary LOBs automatically get freed at the end of a PL/SQL program block. You can also explicitly free the temporary LOBs any time. In OCI and Java, the returned temporary LOB must be freed by the user explicitly.

  Without proper deallocation of the temporary LOBs returned from SQL queries, temporary tablespace gets filled up steadily and you could observe performance degradation.

Performance Considerations for SQL Semantics and LOBs

Be aware of the following performance issues when using SQL semantics with LOBs:

• Ensure that your temporary tablespace is large enough to accommodate LOBs stored out-of-line. Persistent LOBs that are greater than 4K bytes in size are stored outside of the LOB column.
• When possible, free unneeded temporary LOB instances. Unless you explicitly free a temporary LOB instance, the LOB remains in existence while your application is executing. More specifically, the instance exists while the scope in which the LOB was declared is executing.

  See Also: Chapter 9, "SQL Semantics and LOBs" for details on SQL semantics support for LOBs.

Moving Data to LOBs in a Threaded Environment

There two procedures that you can use to move data to LOBs in a threaded environment, one of which should be avoided.

Procedure to Avoid

The following sequence requires a new connection when using a threaded environment, adversely affects performance, and is not recommended:

1. Create an empty (non-NULL) LOB
2. INSERT using the empty LOB
3. SELECT-FOR-UPDATE of the row just entered
4. Move data into the LOB
5. COMMIT. This releases the SELECT-FOR-UPDATE locks and makes the LOB data persistent.

Recommended Procedure

Note: There is no need to create an empty LOB in this procedure. You can use the RETURNING clause as part of the INSERT/UPDATE statement to return a locked LOB locator. This eliminates the need for doing a SELECT-FOR-UPDATE, as mentioned in step 3.

The recommended procedure is as follows:

1. INSERT an empty LOB, RETURNING the LOB locator.
2. Move data into the LOB using this locator.
3. COMMIT. This releases the SELECT-FOR-UPDATE locks, and makes the LOB data persistent.

Alternatively, you can insert >4,000 byte of data directly for the LOB columns but not the LOB attributes.