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Oracle Sequences tips

Oracle Database Tips by Donald Burleson

Proper Use of Oracle Sequences

INST_SEQ_ID:=(INST_ID-1)*100000 + SEQ_NUM;

Generally, sequences can be cached with cache values as high as 200 in RAC. This is much higher than for a regular Oracle instance. If there is insufficient caching, then contention can result which will show up as an increase in service times. If there are performance problems due to sequences, then examine the row cache locks statistics in the v$system_eventview to determine whether the problem is due to the use of Oracle sequences.

Use Oracle sequences to generate unique numbers and set the CACHE parameter to high value, if needed.

Administration of Sequences

Sequences are special database objects that provide numbers in sequence for input to a table. They are useful for providing generated primary key values and for input of number type columns such as purchase order, employee number, sample number, and sales order number, where the input must be unique and in some form of numerical sequence.

Creation of Sequences

Sequences are created by use of the CREATE SEQUENCE command.


sequence_name. The name you want the sequence to have. This may include the user name if created from an account with DBA privilege.

n. An integer, positive or negative.

INCREMENT BY. Tells the system how to increment the sequence. If it is positive, the values are ascending; if it is negative, the values are descending.

START WITH. Tells the system which integer to start with.

MINVALUE. Tells the system how low the sequence can go. For ascending sequences, it defaults to 1; for descending sequences, the default value is 10e27-1.

MAXVALUE. Tells the system the highest value that will be allowed. For descending sequences, the default is 1; for ascending sequences, the default is 10e27-1.

CYCLE. Causes the sequences to automatically recycle to minvalue when maxvalue is reached for ascending sequences; for descending sequences, it causes a recycle from minvalue back to maxvalue.

CACHE. Caches the specified number of sequence values into the buffers in the SGA. This speeds access, but all cached numbers are lost when the database is shut down. The default value is 20; maximum value is maxvalue-minvalue.

ORDER. Forces sequence numbers to be output in order of request. In cases where they are used for timestamping, this may be required. In most cases, the sequences numbers will be in order anyway, so ORDER will not be required. ORDER is necessary only to guarantee ordered generation if you are using Oracle with the Oracle Real Application Clusters option in parallel mode. If you are using exclusive mode, Oracle sequences numbers are always generated in order.

See Altering Oracle sequences

See my notes on Oracle sequences here:




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