SQL & PL/SQL
DB Version: 10.2.0.4
Users: TESTUSR (Table Owner)
TESTUSR_PKG (Package Onwer)
TESTUSR_APP (Application Owner)
create table test_tbl(a number);
CREATE OR REPLACE PACKAGE test_pkg AS -- spec
PROCEDURE insert_test_tbl (
PROCEDURE delete_test_tbl (
CREATE OR REPLACE PACKAGE BODY test_pkg AS -- body
PROCEDURE insert_test_tbl (
Vi is an everyday tool for an Oracle DBA. Commands are many, here's a PDF cheatsheet if you want to become an expert: http://www.math.ntu.edu.tw/~wwang/mtxcomp2010/cssd2011/download/vi_cheat%20sheet.pdf
At the end you get handle yourself with the same commands. I specifically use:
/ -> Lookahead
(with apologies to Robert Ludlum and Eric Van Lustbader)
Oracle performance tuning is an excellent source of myths. The very best ones have a group of adherents who continue to support the myth even when presented with counter-examples. Who’s heard of these?
- Joins are faster than sub-queries
- Sub-queries are faster than joins
- Full Table Scans are bad
Those ones have been around as long as I can remember. Probably the single greatest concentration of Oracle performance tuning myths centres on Bitmap Indexes. Are these familiar?
- Bitmap indexes are good for low-cardinality columns, whereas B-Tree indexes are good for high-cardinality columns.
- Bitmap indexes are slow to update.
- Bitmap indexes don't support concurrent updates.
This is first post of the four-part epic - The Bitmap Conspiracy - detailing the structure and behaviour of Bitmap Indexes. Later in the series we will cover the internal structure of Bitmap Indexes, how Oracle uses them, and finally we will expose some of the myths surrounding them. But before we get there let’s just get a clear understanding of what a Bitmap Index actually is.
I’ve been tuning Oracle database applications for a long time now. I started out recognising some simple patterns and applying template fixes (Got a full table scan? Use an index!) but such a collection of “Do this; don’t do that” anecdotes will only take you so far. If you are curious (I was), you can uncover the reasons why one method is faster than another; i.e. what is the computer doing to make slow code so slow. I found that a good understanding of the internals meant that you didn’t always need to know how to tune a specific example because you could work it out for yourself.
In a database application, these investigations frequently lead to data structures; how does the database store its information and how does it retrieve it? Good information on the internals of Bitmap Indexes is hard to piece together, so in Part 2 of this Bitmap Indexing epic we will look more closely at the internals of Bitmap indexes.
This is Part 3 of The Bitmap Conspiracy, a four part epic on Bitmap Indexes.
In Part 1 we touched briefly on how Oracle can use Bitmap Indexes to resolve queries by translating equality and range predicates into bitmap retrievals. Now that we know more about how they are stored (see Part 2), let’s look closer at some of the operations that Oracle uses to access Bitmap Indexes and manipulate bitmaps.
This is one of those myths that is probably best demonstrated by example. First, let’s create some test data: 10 million rows with the following columns:
- COL1 – Alternating values of 1 and 0 – not indexed
- COL2 – Identical to COL1 – bitmap indexed
- PAYLOAD – 100 bytes of X’s to make the rows a realistic width – not indexed
COL1 and COL2 are identical low cardinality columns: there are only two valid values.
I recently did a comparison caching mechanisms of scalar subquery caching(SSC) and deterministic functions in 11.2. Unfortunately, I do not have enough time to do a full analysis, so I will post it in parts.
1. Both mechanisms are based on hash functions.(About hash tables and hash collisions for scalar subquery caching excelent wrote Tom Kyte and Jonathan Lewis(“Cost-Based Oracle fundamentals” chapter 9))
2. Deterministic caching depends on fetch size(arraysize) – results cached only within one fetch call, ssc has no this limitation.
I want to share my modifications of Tom Kyte's runstats package, which include:
- Any number of runs sets for analyzing
- Standalone: No need to create other objects
- Ability to specify session SID for statistics gathering
- Ability to specify what to gather: latches, stats or both
- Separate mask filters for output by statname and latchname
- Ability to specify difference percentage for output separately for latches and statistics
- More accurate with some statistics because of avoiding global temporary table usage, but less in
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