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Doc: improve documentation about nextval()/setval().

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Clarify that the results of nextval and setval are not guaranteed
persistent until the calling transaction commits.  Some people
seem to have drawn the opposite conclusion from the statement that
these functions are never rolled back, so re-word to avoid saying
it quite that way.

Discussion: https://postgr.es/m/CAKU4AWohO=NfM-4KiZWvdc+z3c1C9FrUBR6xnReFJ6sfy0i=Lw@mail.gmail.com
This commit is contained in:
Tom Lane 2021-11-24 13:37:11 -05:00
parent 373e552189
commit 4ac452e228
1 changed files with 22 additions and 9 deletions
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doc/src/sgml/func.sgml
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@ -17645,24 +17645,37 @@ SELECT setval('myseq', 42, false); <lineannotation>Next <function>nextval</fu
<caution>
<para>
To avoid blocking concurrent transactions that obtain numbers from
the same sequence, a <function>nextval</function> operation is never
rolled back; that is, once a value has been fetched it is considered
used and will not be returned again. This is true even if the
surrounding transaction later aborts, or if the calling query ends
up not using the value. For example an <command>INSERT</command> with
the same sequence, the value obtained by <function>nextval</function>
is not reclaimed for re-use if the calling transaction later aborts.
This means that transaction aborts or database crashes can result in
gaps in the sequence of assigned values. That can happen without a
transaction abort, too. For example an <command>INSERT</command> with
an <literal>ON CONFLICT</literal> clause will compute the to-be-inserted
tuple, including doing any required <function>nextval</function>
calls, before detecting any conflict that would cause it to follow
the <literal>ON CONFLICT</literal> rule instead. Such cases will leave
unused <quote>holes</quote> in the sequence of assigned values.
the <literal>ON CONFLICT</literal> rule instead.
Thus, <productname>PostgreSQL</productname> sequence
objects <emphasis>cannot be used to obtain <quote>gapless</quote>
sequences</emphasis>.
</para>
<para>
Likewise, any sequence state changes made by <function>setval</function>
are not undone if the transaction rolls back.
Likewise, sequence state changes made by <function>setval</function>
are immediately visible to other transactions, and are not undone if
the calling transaction rolls back.
</para>
<para>
If the database cluster crashes before committing a transaction
containing a <function>nextval</function>
or <function>setval</function> 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. This is harmless for usage of the sequence within
the database, since other effects of uncommitted transactions will not
be visible either. However, if you wish to use a sequence value for
persistent outside-the-database purposes, make sure that the
<function>nextval</function> call has been committed before doing so.
</para>
</caution>