Not everybody knows that in SQL Plus the character # (called hash or pound) has a special use that I discovered it myself not much time ago. Well, it’s not really a secret, you can read about it on the official SQL Plus manual, but it’s rarely used. The sign #, in fact, is the default character used as SQLPREFIX during a SQL Plus session and can be changed executing the command
SET SQLPRE[FIX] prefix_char
But what does it do? It tells SQL Plus to execute a SQL Plus command while in the middle of entering a SQL statement or a Pl/Sql block of code.
For example you are writing a select statement
SQL> select empno, ename,
and, while writing down the list of columns to extract, you don’t remember the name of one of them. Normally you would quit writing the query, execute a desc of the table and start writing it again from scratch. But the # sign give us another possibility. All you have to do is enter a new line, digit the magic sign # and write the desc you need
Similarly as discussed in this post explaining how to enable partial unique indexes on Oracle 12c, it is possible to use partial indexes on Oracle 11g (tested on 188.8.131.52.0) as well! As you know this feature has been introduced with the Oracle release 12c but using the same method explained in the previous post we can apply it to 11g.
With release 12c Oracle has introduced partial indexing, the possibility to define partial local indexes on partitioned table. This is an interesting feature that allows to save some space getting rid of index segments for old partitions. Let’s quickly see how this works.
First of all, when creating a partitioned table, the INDEXING property can be specified to define if we want or not to create the indexes for that partition (default value is ON unless a different default is set at table level)
drop table MYTAB;
create table MYTAB(id number(10),
partition by range(run_date)
(partition P1 values less than (date '2016-01-02') INDEXING OFF,
partition P2 values less than (date '2016-01-03'),
partition P3 values less than (date '2016-01-04')
This property can also be changed in a second time issuing a simple alter command
alter table MYTAB move partition P1 indexing on;
The local indexes need to be created with the option INDEXING PARTIAL otherwise they will not be affected by the INDEXING ON/OFF property set at partition/subpartition level.
There is a restriction though: local unique indexes cannot be created as partial so it’s not possible to drop partitions of a unique index. Well, this is at least what Oracle says but let’s go on.
I’m pretty sure that everybody at least once in his professional coding life had the need to convert a string of character separated values into tokens or rows.
Let’s be honest, we all ended writing something like
for k in 1..length(mystring) loop
if substr(mystring), k, 1) = ',' then
strBuffer := strBuffer || substr(mystring, k, 1);
Problem is that among all those “bla” it’s quite easy to forget something (is there always a separator at the very end of the string? Any space between values and separators? Did I clean the buffer?) and in such code bugs might find a comfortable home.
Why should we write such risky code to do something that can be achieved differently in a completely reliable, fast and elegant way? And all of this just using one powerful feature that Oracle kindly introduced with version 10g (drum roll): regular expressions!
Some time ago I came across a query that was using dynamic sampling on a partitioned table even though statistics were present at global level. After some time spent investigating this issue I discovered an interesting but unexpected behaviour of the Oracle CBO when dealing with the GLOBAL_STATS statistic (observed on Oracle RDBMS ver. 184.108.40.206.0, 220.127.116.11.0, 18.104.22.168.0).
Let me go through a simple demo so I can explain what I’ve found.
Welcome to Oracle Findings!
My name is Roberto Rigliaco and as you can easily imagine I’m passionate for database technologies. I’ve been working on Oracle RDBMS for many years and I decided to share my discoveries hoping you will find them as interesting as I do. I just want to clarify that these are my personal findings, things I have learned working on Oracle databases. You might read posts talking about similar topics on other blogs, I’m only reporting and sharing my direct experience.
All your comments are more than welcome.