7 Performance Guidelines

Chunk Size

A chunk is one or more Oracle blocks. You can specify the chunk size for the LOB when creating the table that contains the LOB. This corresponds to the data size used by Oracle Database when accessing or modifying the LOB value. Part of the chunk is used to store system-related information and the rest stores the LOB value. The API you are using has a function that returns the amount of space used in the LOB chunk to store the LOB value. In PL/SQL use DBMS_LOB.GETCHUNKSIZE. In OCI, use OCILobGetChunkSize().

Performance Guidelines for Small 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 CHUNK 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 pieces 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/Check-in 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 is large 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 PL/SQL, use NOCOPY to pass temporary LOB parameters by reference whenever possible. Refer to the Oracle Database 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 PL/SQL programs.

  In PL/SQL, C (OCI), Java and other programmatic interfaces, SQL query results or PL/SQL 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 PL/SQL, 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 3964 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.

  
  

Example of Retrieving LOB Access Statistics

This example was created for retrieving LOB access statistics.

rem
rem Set up the user
rem
 
CONNECT / AS SYSDBA;
SET ECHO ON; 
GRANT SELECT_CATALOG_ROLE TO pm;
GRANT SELECT ON sys.v_$mystat TO pm;
GRANT SELECT ON sys.v_$statname TO pm;
 
rem
rem Create a simplified view for statistics queries
rem
 
CONNECT pm/pm;
SET ECHO ON;
 
DROP VIEW mylobstats;
CREATE VIEW mylobstats
AS
SELECT  SUBSTR(n.name,1,20) name,
        m.value             value
FROM    v$mystat    m,
        v$statname  n
WHERE   m.statistic# = n.statistic#
    AND n.name LIKE 'lob%';
 
rem
rem Create a test table
rem
 
DROP TABLE t;
CREATE TABLE t (i NUMBER, c CLOB)
    lob(c) STORE AS (DISABLE STORAGE IN ROW);
 
rem
rem Populate some data
rem
rem This should result in unaligned writes, one for
rem each row/lob populated.
rem

CONNECT pm/pm;
SELECT * FROM mylobstats;
INSERT INTO t VALUES (1, 'a');
INSERT INTO t VALUES (2, rpad('a',4000,'a'));
COMMIT;
SELECT * FROM mylobstats;
 
rem
rem Get the lob length
rem
rem Computing lob length does not read lob data, no change
rem in read/write stats.
rem
 
CONNECT pm/pm;
SELECT * FROM mylobstats;
SELECT LENGTH(c) FROM t;
SELECT * FROM mylobstats;
 
rem
rem Read the lobs
rem
rem Lob reads are performed, one for each lob in the table.
rem
 
CONNECT pm/pm;
SELECT * FROM mylobstats;
SELECT * FROM t;
SELECT * FROM mylobstats;
 
rem
rem Read and manipulate the lobs (via temporary lobs)
rem
rem The use of complex operators like "substr()" results in
rem the implicit creation and use of temporary lobs. operations
rem on temporary lobs also update lob statistics.
rem
 
CONNECT pm/pm;
SELECT * FROM mylobstats;
SELECT substr(c, length(c), 1) FROM t;
SELECT substr(c, 1, 1) FROM t;
SELECT * FROM mylobstats;
 
rem
rem Perform some aligned overwrites
rem
rem Only lob write statistics are updated since both the
rem byte offset of the write, and the size of the buffer
rem being written are aligned on the lob chunksize.
rem
 
CONNECT pm/pm;
SELECT * FROM mylobstats;
DECLARE
    loc     CLOB;
    buf     LONG;
    chunk   NUMBER;
BEGIN
    SELECT c INTO loc FROM t WHERE i = 1
        FOR UPDATE;
 
    chunk := DBMS_LOB.GETCHUNKSIZE(loc);
    buf   := rpad('b', chunk, 'b');
 
    -- aligned buffer length and offset
    DBMS_LOB.WRITE(loc, chunk, 1, buf);
    DBMS_LOB.WRITE(loc, chunk, 1+chunk, buf);
    COMMIT;
END;
/
SELECT * FROM mylobstats;
 
rem
rem Perform some unaligned overwrites
rem 
rem Both lob write and lob unaligned write statistics are
rem updated since either one or both of the write byte offset
rem and buffer size are unaligned with the lob's chunksize.
rem 
 
CONNECT pm/pm;
SELECT * FROM mylobstats;
DECLARE
    loc CLOB;
    buf LONG;
BEGIN
    SELECT c INTO loc FROM t WHERE i = 1
        FOR UPDATE;
 
    buf := rpad('b', DBMS_LOB.GETCHUNKSIZE(loc), 'b');
 
    -- unaligned buffer length
    DBMS_LOB.WRITE(loc, DBMS_LOB.GETCHUNKSIZE(loc)-1, 1, buf);
 
    -- unaligned start offset
    DBMS_LOB.WRITE(loc, DBMS_LOB.GETCHUNKSIZE(loc), 2, buf);
 
    -- unaligned buffer length and start offset
    DBMS_LOB.WRITE(loc, DBMS_LOB.GETCHUNKSIZE(loc)-1, 2, buf);
 
    COMMIT;
END;
/
SELECT * FROM mylobstats;
DROP TABLE t;
DROP VIEW mylobstats;
 
CONNECT / AS SYSDBA
REVOKE SELECT_CATALOG_ROLE FROM pm;
REVOKE SELECT ON sys.v_$mystat FROM pm;
REVOKE SELECT ON sys.v_$statname FROM pm;
 
QUIT;