Q1: What are the default values for
an Oracle sequence when no parameters are specified?
A1: When you create a sequence without specifying any parameters, Oracle
uses the following default values:
- START WITH = 1 (sequence starts from 1)
- INCREMENT BY = 1 (increments by 1 each time)
- MINVALUE = 1 (minimum value of 1 for ascending sequences)
- MAXVALUE = 10^27 (very large maximum value for ascending sequences)
- CYCLE = NOCYCLE (does not restart the sequence after reaching the maximum value)
- CACHE = 20 (pre-allocates 20 numbers for better performance)
- ORDER = NOORDER (sequence numbers are not guaranteed to be generated in strict order in RAC environments)
Q2: What does START WITH default to in an Oracle sequence?
A2: The default value for START
WITH is 1. This means the sequence will begin
generating numbers starting from 1 unless specified otherwise.
Q3: Can I change the default
increment value in Oracle sequences?
A3: Yes, the default increment value is 1. You can change it by specifying
the INCREMENT BY clause during sequence creation:
CREATE SEQUENCE my_seq INCREMENT BY 5;
This would make the sequence increment by 5 instead of the default 1.
Q4: What is the default MINVALUE for a sequence in Oracle?
A4: The default MINVALUE for ascending sequences is 1.
This is the minimum value the sequence can generate unless specified otherwise.
Q5: What is the default MAXVALUE in Oracle sequences?
A5: By default, the MAXVALUE for ascending sequences is set to 10^27,
which is a very large number (10 raised to the power of 27). This is the upper
limit unless a different value is specified.
Q6: Does Oracle sequence cycle by
default?
A6: No, the default setting for Oracle sequences is NOCYCLE,
which means the sequence will not restart once it reaches the maximum value. To
allow the sequence to restart, you would need to explicitly set CYCLE.
Q7: What is the default CACHE
size for Oracle sequences?
A7: The default cache size for a sequence in Oracle is 20.
This means Oracle will pre-allocate 20 sequence values to improve performance.
If you want to change this, you can specify a different cache size during
sequence creation.
Q8: What happens if I don’t specify
the ORDER clause?
A8: If you don't specify the ORDER clause, Oracle will default to NOORDER.
This means that sequence numbers are not guaranteed to be generated in strict
order, which is generally the case unless you are using Oracle Real Application
Clusters (RAC).
Q9: Can I modify the default values
of a sequence after it is created?
A9: No, you cannot directly modify a sequence after it is created.
However, you can drop and recreate the sequence with the desired values if you
need to change its behavior (such as START
WITH, INCREMENT
BY, etc.).
Q10: Does Oracle guarantee the order
of sequence values in a RAC environment?
A10: By default, Oracle does not guarantee the order of sequence values
across different nodes in a Real Application Clusters (RAC) environment, as the
default setting is NOORDER. If you need guaranteed ordering, you
can use the ORDER clause, but keep in mind this can
impact performance.
Q11: What happens if I specify both CACHE
and NOCACHE?
A11: You cannot use both CACHE and NOCACHE together. If you specify CACHE,
Oracle will pre-allocate a number of sequence values for better performance. If
you specify NOCACHE, it will generate each sequence
number on-demand, which may cause performance issues due to disk access.
Q12: What does ALWAYS mean in the context of identity columns?
A12: For identity columns (which are backed by sequences), the default
behavior is ALWAYS. This means the sequence will
automatically generate a value for the identity column whenever a row is
inserted, and you cannot manually insert a value into that column.
Q13: What is the default behavior if
no sequence options are specified in Oracle?
A13: If no options are specified when creating a sequence, Oracle will
use the default values for all the parameters:
- START WITH = 1
- INCREMENT BY = 1
- MINVALUE = 1
- MAXVALUE = 10^27
- CYCLE = NOCYCLE
- CACHE = 20
- ORDER = NOORDER
These default values are generally sufficient for many use cases unless you have specific requirements.
Q14: Can I customize the behavior of
a sequence during creation?
A14: Yes, you can customize various sequence behaviors during creation
by specifying parameters such as START
WITH, INCREMENT
BY, MINVALUE, MAXVALUE, CYCLE, CACHE, and ORDER.
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