Sequence Default Values FAQS

 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.