blog.watashii » performance

Vacuum Database to Speedup Firefox

watashii — Tue, 25 Aug 2009 05:37:15 +0000

Firefox performance can be improved by defragmenting the internal SQLite database. This can be done by running a vacuum command within Firefox, as explained in Mozilla Links. The process are as follows:

1) Open Tools > Error Console (or Press Ctrl + Shift + J)

2) In the Code text box, paste the following (in one single line)

Components.classes["@mozilla.org/browser/nav-history-service;1"].
getService(Components.interfaces.nsPIPlacesDatabase).
DBConnection.executeSimpleSQL("VACUUM");

Note: Make sure the double-quotes are normal quotes and not slanted quotes

3) Press Evaluate. Firefox will freeze for a few seconds while the database is being defragmented.

A handier add-on extension is also available to perform this action.

Exercise Your Idle System Doing Nothing

watashii — Thu, 30 Apr 2009 15:15:49 +0000

Via Unix/Linux:

cat /dev/zero > /dev/null

cp /dev/zero  /dev/null

Explaination:

/dev/null is basically a black hole (a special null file), anything written to it goes down the drain. Anything read from it will result in nothing returned. /dev/zero is basically the same as /dev/null when writing to it, however reading it will result in a continuous stream of zeros without EOF.

Oracle Index Null Values

watashii — Sun, 21 Sep 2008 12:02:39 +0000

When creating Oracle indexes, Oracle ignores the NULL values. For example, the following index was created over “emp_name” column.

create index emp_name_idx
on employees (emp_name);

Querying for a NULL value on the indexed column would yield a full table scan.

select emp_name from employees where emp_name is NULL;

So how can we make Oracle perform a fast indexed search?

The solution is to index NULL values with a function-based index. The following example replaces all the NULL values with a string named ‘null’ (note, this can be any arbitrary string). This allows the NULL to be included as if it was a real value.

create index emp_name_idx
on employees (nvl(emp_name,'null'));

Viewing Firefox Cache Files

watashii — Sat, 09 Aug 2008 08:55:42 +0000

The browser cache stores all the web pages and images downloaded on to the computer to allow faster access and reduce the downloading bandwidth when revisiting a website.

Type “about:cache” on your Firefox address bar to view the cached files. There are 3 types of cache for allocating the files…

1) Memory cache device, stores all the files which are downloaded per session into your computers RAM (or temporary memory). This means after you restart your Firefox, the files will be cleared out.

2) Disk cache device, stores all the files into your hard disk drive. This is some file location in your computer, for example within your Windows C:\ drive. These are available after restarting the Firefox and accessible without an internet connection.

3) Offline cache device, stores all the files which can be used for offline browsing. Typically it is empty, but is targeted for upcoming offline applications. Watch this space.

One useful tip to improve FIrefox performance is to disable disk cache (or set Tools > Options > Advanced > Network > Offline Storage: Use 0Mb). What this means to to purely rely on the RAM memory for cached data, and not read from the disk. A simple concept: memory read speed is faster than disk read speed.

PL/SQL – Pipelined Table Function

watashii — Wed, 02 Jul 2008 13:16:33 +0000

Normally when writing PL/SQL functions, they return a single value result. But what if you want to return a collection type instead? The easiest way to do this is via a pipelined function, with the PIPELINED keyword, in which rows are returned by the function iteratively. It can be created at the schema level, or inside a package. I prefer the latter since its more portable, and easier to manage.

Inside the function, you return individual elements of the collection type, instead of returning the entire collection type when function ends. This also has performance benefits, depending on your application.

The code below defines a custom number collection type and a pipelined function; within a package / package body.

CREATE PACKAGE mypackage AS
  TYPE num_type IS TABLE OF NUMBER;
  FUNCTION myfunction (x NUMBER) RETURN num_type PIPELINED;
END mypackage;
/

CREATE PACKAGE BODY mypackage AS
  -- myfunction returns a collection of elements (1,2,3,... x)
  FUNCTION myfunction (x NUMBER) RETURN num_type PIPELINED IS
    BEGIN
      FOR i IN 1..x LOOP
        -- each number is returned per iteration
        PIPE ROW(i);
      END LOOP;
      RETURN;
    END;
END mypackage;
/

To call the function, simply call:

SELECT * FROM TABLE(mypackage.myfunction(5));

Which results in:
COLUMN_VALUE ------------ 1 2 3 4 5