Distributing Named Sequences
When you create a named sequence, its
CACHE parameter determines the number of sequence values each node maintains during a session. The default cache value is 250K, so each node reserves 250,000 values per session for each sequence. The default cache size provides an efficient means for large insert or copy operations.
If sequence caching is set to a lower number, nodes are liable to request a new set of cache values more frequently. While it supplies new cache, Vertica must lock the catalog. Until Vertica releases the lock, other database activities such as table inserts are blocked, which can adversely affect overall performance.
When a new session starts, node caches are initially empty. By default, the initiator node requests and reserves cache for all nodes in a cluster. You can change this default so each node requests its own cache, by setting configuration parameter
ClusterSequenceCacheMode to 0.
For information on how Vertica requests and distributes cache among all nodes in a cluster, refer to Sequence Caching.
Effects of Distributed Sessions
Vertica distributes a session across all nodes. The first time a cluster node calls the function
NEXTVAL on a sequence to increment (or decrement) its value, the node requests its own cache of sequence values. The node then maintains that cache for the current session. As other nodes call
NEXTVAL, they too create and maintain their own cache of sequence values.
During a session, nodes call
NEXTVAL independently and at different frequencies. Each node uses its own cache to populate the sequence. All sequence values are guaranteed to be unique, but can be out of order with a
NEXTVAL statement executed on another node. As a result, sequence values are often non-contiguous.
In all cases, increments a sequence only once per row. Thus, if the same sequence is referenced by multiple columns,
NEXTVAL sets all columns in that row to the same value. This applies to rows of joined tables.
Calculating Named Sequences
Vertica calculates the current value of a sequence as follows:
- At the end of every statement, the state of all sequences used in the session is returned to the initiator node.
- The initiator node calculates the maximum
CURRVALof each sequence across all states on all nodes.
- This maximum value is used as
CURRVALin subsequent statements until another
Losing Sequence Values
Sequence values in cache can be lost in the following situations:
- If a statement fails after
NEXTVALis called (thereby consuming a sequence value from the cache), the value is lost.
- If a disconnect occurs (for example, dropped session), any remaining values in cache that have not been returned through NEXTVAL are lost.
- When the initiator node distributes a new block of cache to each node where one or more nodes has not used up its current cache allotment. For information on this scenario, refer to Sequence Caching.
You can recover lost sequence values by using
ALTER SEQUENCE...RESTART, which resets the sequence to the specified value in the next session.
ALTER SEQUENCE to set a sequence start value below its current value can result in duplicate keys.