To allocate cache among the nodes in a cluster for a given sequence, Vertica uses the following process:
- By default, when a session begins, the cluster initiator node requests cache for itself and other nodes in the cluster.
- The initiator node distributes cache to other nodes when it distributes the execution plan.
- Because the initiator node requests caching for all nodes, only the initiator locks the global catalog for the cache request.
Example:
dbadmin=> CREATE SEQUENCE jim_seq CACHE 5;
CREATE SEQUENCE
dbadmin=> SELECT sequence_name, left FROM session_sequences;
sequence_name | left
---------------+------
(0 rows)
dbadmin=> SELECT jim_seq.NEXTVAL;
NEXTVAL
---------
1
(1 row)
dbadmin=> SELECT sequence_name, left FROM session_sequences;
sequence_name | left
---------------+------
jim_seq | 4
(1 row)
dbadmin=> SELECT jim_seq.NEXTVAL;
NEXTVAL
---------
2
(1 row)
dbadmin=> SELECT jim_seq.NEXTVAL;
NEXTVAL
---------
3
(1 row)
dbadmin=> SELECT sequence_name, left FROM session_sequences;
sequence_name | left
---------------+------
jim_seq | 2
(1 row)
dbadmin=> SELECT jim_seq.NEXTVAL;
NEXTVAL
---------
4
(1 row)
dbadmin=> SELECT jim_seq.NEXTVAL;
NEXTVAL
---------
5
(1 row)
dbadmin=> SELECT sequence_name, left FROM session_sequences;
sequence_name | left
---------------+------
jim_seq | 0
(1 row)
dbadmin=> SELECT jim_seq.NEXTVAL;
NEXTVAL
---------
6
(1 row)
dbadmin=> SELECT sequence_name, left FROM session_sequences;
sequence_name | left
---------------+------
jim_seq | 4
(1 row)
Helpful Links:https://www.vertica.com/docs/latest/HTML/Content/Authoring/AdministratorsGuide/Sequences/Sequences.htm https://www.vertica.com/docs/latest/HTML/Content/Authoring/AdministratorsGuide/Sequences/HowVerticaAllotsCacheforSequencing.htm
Have fun!
About the Author
James Knicely
Vertica Field Chief Technologist
I've had the privilege of working with many database technologies in my career. But after being introduced to Vertica in May of 2011 as a client, I was hooked on the new technology after witnessing a query run in milliseconds that had previously ran for hours on the legacy database we had in place. It was then that I knew I wanted to eventually join the Vertica team, and 4 years later I did! I am currently a Vertica evangelist and am ready to help you get on board! Please feel free to reach out to me with any questions you have about Vertica and make sure to follow my Vertica Quick Tips!
