I have been neglecting my blog recently and decided to devote a page to some of the small things I have done. They are not necessarily about how something works instead some are showing syntax and others useful things. Also this is a reminder for myself something. I will be adding to this page with more stuff.
Oracle - CREATE TABLE <name> AS........
The Oracle Recycle Bin
Oracle - Getting the DDL for an object
Corrupt UNDO segment.
Loading Large Tables with a PK
Oracle - CREATE TABLE <name> AS........
Using Oracle's CREATE TABLE <name> AS SELECT * FROM syntax creates a replica of a table but does not preserve partition, primark keys or indexes.
MySQL uses the CREATE TABLE <name> LIKE... which copies the structure.
The Oracle Recycle Bin
Whats in it
select object_name, original_name, type, can_undrop, can_purge, droptime
from recyclebin;
Getting tables back
flashback table t1 to before drop;
Getting specific tables back
flashback table “<recycle bin name>″ to before drop;
Turning the recyclebin off
ALTER SESSION SET RECYCLEBIN=OFF
Ref - http://www.orafaq.com/node/968
Oracle - Getting the DDL for an object
MySQL, MariaDB and other variants have a great way of getting the DDL for an object - SHOW CREATE TABLE <name>. Oracle doesn't have the same facility but you can use DBMS_METADATA:
SQL> SET long 9999999
SQL> select DBMS_METADATA.get_ddl ('TABLE','TESTTAB') from user_tables where table_name = 'TESTTAB';
For indexes:
SQL> select DBMS_METADATA.get_ddl ('INDEX','TESTTAB_IDX') from user_indexes where table_name = 'TESTTAB';
Corrupt UNDO segment.
I have never seen this before but after what I can only think was a session that had been aborted the rbs segment was corrupt:
11:25:17 SQL> select segment_name,status,tablespace_name from dba_rollback_segs where status='NEEDS RECOVERY';
SEGMENT_NAME STATUS TABLESPACE_NAME
------------------------------ ---------------- ------------------------------
_SYSSMU8_2012382730$ NEEDS RECOVERY UNDOTBS1
I was not sure how to rectify this but I noticed the database took over 5 minutes to shutdown. When tailing the alert.log it would look like it was hung after the system did the following:
Mon May 12 11:47:32 2014
SMON: slave died unexpectedly, downgrading to serial recovery
ALTER DATABASE CLOSE NORMAL
Mon May 12 11:51:32 2014
I found a great post that gave me some good tips on solving the issue
http://mikegeorgiou.blogspot.co.uk/2008/06/drop-corrupted-undo-tablespace-with.html
Loading Large Tables with a PK
Recently I had to load 111 million rows into an Oracle table from another table. The table was pre-created and I ran a simple INSERT INTO...SELECT * FROM command. This was taking a long time (10 hours +) so I dropped the PK and rerun the statement. It was much quicker. Next I need to add the PK. I created the unique index first (using the parallel clause) and then ran an alter table to add the constraint. This allowed me to create the PK with a parallel clause which is far quicker.
create unique index pk_index_name on <table> (col1, col2...) parallel 4;
Took 40 mins
ALTER TABLE <table_name> ADD CONSTRAINT pk_con_name PRIMARY KEY (col1, col2...) using index pk_index_name;
Took 1 second.
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