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[Blog] Oracle SGA [Original] 2008-9-18 5:51pm

The main components of the SGA are the database buffer cache, the shared pool, the redo log buffer, the Java pool, and the optional large pool.

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[Blog] what is ACID? [Original] 2008-9-1 12:36pm

Transactional systems often are described as being ACID compliant.

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[Blog] Locking Issues [Original] 2008-8-29 12:48am

If a major table or tables are locked unbeknownst to you, the database could slow down dramatically in very short order.

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[Blog] Supplemental Logging in a Streams Environment [Original] 2008-8-28 5:50pm

Supplemental logging places additional column data into a redo log whenever an UPDATE operation is performed. Such updates include piecewise updates to LOBs. The capture process captures this additional information and places it in LCRs.

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[Blog] Delete Duplicate Rows From an Oracle Table [Original] 2008-8-21 12:55pm

It's easy to introduce duplicate rows of data into Oracle tables by running a data load twice without the primary key or unique indexes created or enabled.

Here's how you remove the duplicate rows before the primary key or unique indexes can be created:

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[Blog] V$LOGMNR_CONTENTS [Original] 2008-8-19 11:47pm

LogMiner presents the information in the redo logs through the V$LOGMNR_CONTENTS fixed view.

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[Blog] Database Wait Statistics [Copied] 2008-8-6 12:46pm

1. Summary

High buffer cache hit ratios don't necessarily mean that the application is well tuned. All a high buffer cache hit ratio is telling you is that your physical reads are very small when compared to the total logical reads from the buffer cache. It's entirely possible for you to have a 99.0 buffer cache hit ratio and still have a database slowdown.

 

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[Blog] Implicit cursors & Explicit Cursors [Modified] 2008-8-5 2:00pm

For every SQL statement execution certain area in memory is allocated. PL/SQL allow you to name this area. This private SQL area is called context area or cursor. A cursor acts as a handle or pointer into the context area. A PL/SQL program controls the context area using the cursor. Cursor represents a structure in memory and is different from cursor variable.

Implicit cursors

For SQL queries returning single row PL/SQL declares implicit cursors. Implicit cursors are simple SELECT statements and are written in the BEGIN block (executable section) of the PL/SQL. Implicit cursors are easy to code, and they retrieve exactly one row. PL/SQL implicitly declares cursors for all DML statements. The most commonly raised exceptions here are NO_DATA_FOUND or TOO_MANY_ROWS.

Syntax:

SELECT column_name INTO :block.field_name FROM table_name;

Explicit Cursors

Explicit cursors are used in queries that return multiple rows. The set of rows fetched by a query is called active set. The size of the active set meets the search criteria in the select statement. Explicit cursor is declared in the DECLARE section of PL/SQL program.

Syntax:

CURSOR <cursor-name> IS <select statement>

Sample Code:

DECLARE
CURSOR emp_cur IS SELECT ename FROM EMP;
BEGIN
----
---

END;

Notes: The syntax for implicit cursors is easier to use yet often, less efficient. An explicit cursor is DECLARE'd OPEN'ed FETCH'ed and CLOSE'ed. Note that the user has to explicitly define each step.

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[Blog] SQL Trace and TKPROF [Copied] 2008-8-4 1:10pm

1. If your application has a lot of dynamically generated SQL, the SQL Trace utility is ideal for tuning the SQL statements.

2. SQL Trace will enable you to track the following variables:

* CPU and elapsed times
* Parsed and executed counts for each SQL statement
* Number of physical and logical reads
* Execution plan for all the SQL statements
* Library cache hit ratios

3. There are three parameters related to SQL Trace: Statistics_Level, Timed_Statistics, User_Dump_Dest

Timed_Statistics

To collect performance statistics such as CPU and execution time, set the value of the timed_statistics parameter to true in the init.ora file or SPFILE, or use the alter system set timed_statistics = true statement to turn timed statistics on instancewide. You can also do this at the session level by using the alter session statement as follows:

SQL> alter session set timed_statistics = true;
Session altered

4. Enabling SQL Trace

You can do at the instance level by using the alter session statement or the DBMS_SESSION package.

1) including the line sql_trace=true in your init.ora file or SPFILE or by using the alter system command to set sql_trace to true.

2) SQL> alter session set sql_trace=true; Session altered.

3) DBMS_SESSION.set_sql_trace


5. Interpreting the Trace Files with TKPROF

$ tkprof trace_file.trc trace.prf sys=no explain =y

TKPROF Command-Line Arguments

PARAMETER DESCRIPTION
==========================================================
Filename -- The input trace file produced by SQL Trace

Explain -- The explain plan for the SQL statements

Record -- Creates a SQL script with all the nonrecursive SQL statements

Waits -- Records a summary of wait events

Sorts Sorts the statements in descending order

Table -- The name of the tables into which the TKPROF utility temporarily puts the execution plans

Sys -- Enables and disables listing of SQL statements issued by SYS

Print -- Lists only a specified number of SQL statements instead of all statements

Insert -- Creates a script that stores the trace information in the database

6. Examining the Formatted Output File

count = number of times OCI procedure was executed

cpu = cpu time in seconds executing elapsed = elapsed time in seconds executing

disk = number of physical reads of buffers from disk

query = number of buffers gotten for consistent read

current = number of buffers gotten in current mode (usually for update)

rows = number of rows processed by the fetch or execute call

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[Blog] How to install Statspack? [Modified] 2008-8-4 11:49am
1. connect as SYS and run the installation scripts from the $ORACLE/HOME/rdbms/admin directory.

2. To install Statspack, you need to run three scripts: spcuser.sql, spctab.sql, and spcpkg.sql. The three scripts perform the following tasks:

Spcuser.sql creates the perfstat user.

Spctab.sql creates the necessary tables and other objects to store the statistics.

Spcpkg.sql creates the Statspack package.

3. You need to run the three scripts under different schemas. You must run the first script, spcusr.sql, under the SYS schema, and you must run the next two as the Statspack user perfstat.

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