PostgreSQL sequence create alter nextval Curval numerical interval gap

Syntax

Suppose I want to create sequence The name is seq_for_user_id,：

--  New sequence 
create sequence seq_for_user_id increment by 1 min value 1 start with 1 cache 1 cycle;

--  Modify the sequence 
alter sequence if exists seq_for_user_id increment by 1 minvalue 1 start with 1 cache 100 cycle;

--  Common operations 
select nextval('seq_for_user_id');	--  Get the next value 
select currval('seq_for_user_id');  --  Check the current value ( The last value generated and obtained )

about nextval and currval function , If you report an error xxx The object does not exist in the current session , Can be in sequence name And before schema name.
Such as ：
select nextval('wuyujin1997_demodb.seq_for_user_id');

Document

– newly build sequence

https://www.postgresql.org/docs/current/sql-createsequence.html

CREATE [ TEMPORARY | TEMP ] SEQUENCE [ IF NOT EXISTS ] name
    [ AS data_type ]
    [ INCREMENT [ BY ] increment ]
    [ MINVALUE minvalue | NO MINVALUE ] [ MAXVALUE maxvalue | NO MAXVALUE ]
    [ START [ WITH ] start ] [ CACHE cache ] [ [ NO ] CYCLE ]
    [ OWNED BY { table_name.column_name | NONE } ]

– modify sequence

https://www.postgresql.org/docs/10/sql-altersequence.html

ALTER SEQUENCE [ IF EXISTS ] name
    [ AS data_type ]
    [ INCREMENT [ BY ] increment ]
    [ MINVALUE minvalue | NO MINVALUE ] [ MAXVALUE maxvalue | NO MAXVALUE ]
    [ START [ WITH ] start ]
    [ RESTART [ [ WITH ] restart ] ]
    [ CACHE cache ] [ [ NO ] CYCLE ]
    [ OWNED BY { table_name.column_name | NONE } ]
ALTER SEQUENCE [ IF EXISTS ] name OWNER TO { new_owner | CURRENT_USER | SESSION_USER }
ALTER SEQUENCE [ IF EXISTS ] name RENAME TO new_name
ALTER SEQUENCE [ IF EXISTS ] name SET SCHEMA new_schema

– sequence Correlation function

https://www.postgresql.org/docs/current/functions-sequence.html

nextval(regclass) --> bigint
setval(regclass, bigint[, boolean]) --> bigint
currval(regclass) --> bigint
lastval(regclass) --> bigint

postgresql sequence gap The gap

https://www.postgresql.org/docs/current/functions-sequence.html mention ：
Postgresql Of sequence Cannot be used to obtain “ There is no interval / Numerical sequence of gaps ”.

Caution
To avoid blocking concurrent transactions that obtain numbers from the same sequence, 
the value obtained by nextval is not reclaimed for re-use if the calling transaction later aborts. 
 For the wrong “ Concurrent transactions from the same sequence Get the value in ” This caused a blockage , Those have been used nextval Value obtained by method , Even if the transaction is aborted, it will not be recycled .
This means that transaction aborts or database crashes can result in gaps in the sequence of assigned values. 
 This means that the transaction is aborted / Database crash can cause “ By the same sequence Generated numerical sequence ” Yes, there is a gap / Spaced .
That can happen without a transaction abort, too. 
 This phenomenon can also occur without transaction suspension .
For example an INSERT with an ON CONFLICT clause will compute the to-be-inserted tuple, including doing any required nextval calls, 
before detecting any conflict that would cause it to follow the ON CONFLICT rule instead. 
 for instance , You wrote a  insert...on conflict  The sentence of , The partial values to be inserted are from  sequence  Generate .
 Because of calling  sequence nextval  The latest value is generated , And then do it  insert , If there is a conflict in the insertion action , Insert failed .
 But it has been used nextval The generated value will not be recycled .
Thus, PostgreSQL sequence objects cannot be used to obtain “gapless” sequences.
 therefore ,Postgresql  Of  sequence  Cannot be used to obtain “ There is no interval / Numerical sequence of gaps ”.

Likewise, sequence state changes made by setval are immediately visible to other transactions, and are not undone if the calling transaction rolls back.
 Same as , call  setval  Yes  sequence  The change caused by the state is also immediately visible to other transactions , And even if the transaction is rolled back , Status changes will not be canceled .

If the database cluster crashes before committing a transaction containing a nextval or setval call, 
the sequence state change might not have made its way to persistent storage, 
so that it is uncertain whether the sequence will have its original or updated state after the cluster restarts. 
 If the database cluster is “ Submit a containing  nextval/setval  Called transaction ” Collapsed before , Now  sequence  The state change of may not be persisted to the database server (database server-side), So when the database crashes - After reboot , I'm not sure  sequence  The value of is the old value before changing , Or the updated finger ( It depends on how tiny the database crashed , Whether the change is persistent to DB layer ).
This is harmless for usage of the sequence within the database, since other effects of uncommitted transactions will not be visible either. 
 This is harmless for using sequences in the database , Because other effects of uncommitted transactions will also be invisible .
However, if you wish to use a sequence value for persistent outside-the-database purposes, 
make sure that the nextval call has been committed before doing so.
 however , If you want to use sequence values for persistent off database purposes , Please make sure nextval The call was submitted before this operation .