Estimate database usage - Oracle

One question always haunts me a bit, "Could you provide the database machine requirement?".
Here's one method I often use to estimate the size for Oracle database:
1.  SELECT SUM(BYTES) FROM (
select SEGMENT_NAME, SUM(BYTES)/(1024*1024) BYTES from user_extents
where segment_type = 'TABLE'
and segment_name like 'ACT_%'
GROUP BY SEGMENT_NAME);

select SEGMENT_NAME, SUM(BYTES)/(1024*1024) BYTES from user_extents
where segment_type = 'TABLE'
and segment_name like 'ACT_%'
GROUP BY SEGMENT_NAME;

Find blocking query - Oracle

If you have a query that stops and it seems that it will never end, you might have another query blocking it. Here's one help to find out which query blocking it.

SELECT s1.username || '@' || s1.machine
    || ' ( SID=' || s1.sid || ' )  is blocking '
    || s2.username || '@' || s2.machine || ' ( SID=' || s2.sid || ' ) ' AS blocking_status
    FROM v$lock l1, v$session s1, v$lock l2, v$session s2
    WHERE s1.sid=l1.sid AND s2.sid=l2.sid
    AND l1.BLOCK=1 AND l2.request > 0
    AND l1.id1 = l2.id1
    AND l1.id2 = l2.id2;

References:
http://www.oraclerecipes.com/monitoring/find-blocking-sessions/

Oracle query to find out table basic information

Query below helped me to know the current status of my tables and their places in the oracle db. So that I could further decide whether I should move it or not, notwithstanding that I must understand the undergoing code accessing those tables.

select tbl.OWNER || '.' || tbl.TABLE_NAME table_name, tbl.TABLESPACE_NAME, trunc(ds.BYTES/1024/1024) || ' Mb' table_size, tbl.NUM_ROWS, tbl.LAST_ANALYZED from SYS.ALL_TABLES tbl
join SYS.dba_segments ds on ds.SEGMENT_NAME = tbl.TABLE_NAME
where ds.SEGMENT_TYPE = 'TABLE'

The output of the query will have:
- Table name
- Tablespace where the table is stored
- The size of the table in Mb
- Number of rows
- Last analyzed

The above query could be expanded further just as needed.

How to search for column reference in oracle

Imagine that you need to find out which code update a column in a table. Some developer tools could point out who refers to the table but so far I couldn't find the one that could answer my need.

All_source comes to the rescue, I'd say that it is a special view that holds all the database objects accessible by the user.

Here is the description on the view's columns:

OWNER	VARCHAR2(30)	NOT NULL	Owner of the object
NAME	VARCHAR2(30)	NOT NULL	Name of the object
TYPE	VARCHAR2(12)		Type of object: FUNCTION, JAVA SOURCE, PACKAGE, PACKAGE BODY, PROCEDURE, TRIGGER, TYPE, TYPE BODY
LINE	NUMBER	NOT NULL	Line number of this line of source
TEXT	VARCHAR2(4000)		Text source of the stored object

Example:
I need to find out who refers to column FK_USER.

select * from all_source where text like '%FK_USER%'

And I will get all the places which have reference to FK_USER.

If you would take it further, you could utilize it for searching almost anything (e.g. comments in the code, TODO tag, exception).

... wondering if there're more "magical" view like this.