Histograms

Histograms tell the Optimizer about the distribution of data within a column. By default (without a histogram), the Optimizer assumes a uniform distribution of rows across the distinct values in a column. Therefore, the Optimizer calculates the cardinality (number of rows returned) for an equality predicate by dividing the total number of rows in the table by the number of distinct values in the column used in the equality predicate.

If the data distribution in that column is not uniform (i.e., a data skew exists) then the cardinality estimate will be incorrect.
In order to accurately reflect a non-uniform data distribution, a histogram is required on the column. The presence of a histogram changes the formula used by the Optimizer to estimate the cardinality, and allows it to generate a more accurate execution plan.
A histogram sorts values into “buckets,” as you might sort coins into buckets.

Continue reading Histograms →

The ADR Command Interpreter (ADRCI) is a command-line tool that you use to manage Oracle Database diagnostic data and is part of the fault diagnosability infrastructure introduced in Oracle 11g. ADRCI enables you to:

• View diagnostic data within the Automatic Diagnostic Repository (ADR).
• View Health Monitor reports.
• Package incident and problem information into a zip file for transmission to Oracle Support.

Diagnostic data includes incident and problem descriptions, trace files, dumps, health monitor reports, alert log entries, and more.
ADRCI has a rich command set, and can be used in interactive mode or within scripts. In addition, ADRCI can execute scripts of ADRCI commands in the same way that SQL*Plus executes scripts of SQL and PL/SQL commands.

Invoke ADRCI as the Oracle OS user:

> adrci
ADRCI: Release 11.2.0.3.0 - Production on Fri Jan 17 22:38:57 2014
Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.

ADR base = "/opt/oracle" adrci> show home ADR Homes:
diag/diagtool/user_oracle/host_12454656_11
diag/rdbms/orcl/ORCL
diag/asm/+asm/+ASM1
diag/tnslsnr/orcl/listener_cad Set the home adrci> set home diag/rdbms/orcl/ORCL

To check the alert log use the following option.

adrci> show alert -tail -f
2014-01-21 10:20:23.418000 -05:00
Shutting down archive processes
Archiving is disabled
Archive process shutdown avoided: 0 active
Thread 1 closed at log sequence 1176
Successful close of redo thread 1
Completed: alter database close normal
alter database dismount
Completed: alter database dismount
ARCH: Archival disabled due to shutdown: 1089
Shutting down archive processes
Archiving is disabled
Archive process shutdown avoided: 0 active
2014-01-21 10:20:24.852000 -05:00
ARCH: Archival disabled due to shutdown: 1089
Shutting down archive processes
Archiving is disabled
Archive process shutdown avoided: 0 active
Stopping background process VKTM:
2014-01-21 10:20:27.036000 -05:00
Instance shutdown complete

Continue reading Using ADRCI Properly →