storage parameters. All indexes accept the following parameter:

</para>

<variablelist>

<varlistentry>

<term><literal>recheck_on_update</literal></term>

<listitem>

<para>

Specifies whether to recheck a functional index value to see whether

we can use a HOT update or not. The default value is on for functional

indexes with an total expression cost less than 1000, otherwise off.

You might decide to turn this off if you knew that a function used in

an index is unlikely to return the same value when one of the input

columns is updated and so the recheck is not worth the additional cost

of executing the function.

</para>

<para>

Functional indexes are used frequently for the case where the function

returns a subset of the argument. Examples of this would be accessing

part of a string with <literal>SUBSTR()</literal> or accessing a single

field in a JSON document using an expression such as

<literal>(bookinfo-&gt;&gt;'isbn')</literal>. In this example, the JSON

document might be updated frequently, yet it is uncommon for the ISBN

field for a book to change so we would keep the parameter set to on

for that index. A more frequently changing field might have an index

with this parameter turned off, while very frequently changing fields

might be better to avoid indexing at all under high load.

</para>

</listitem>

</varlistentry>

</variablelist>

<para>

The B-tree, hash, GiST and SP-GiST index methods all accept this parameter:

<listitem>

<!--

2018-03-27 [c203d6cf8] Allow HOT updates for some expression indexes

-->

<para>

Allow heap-only-tuple (<acronym>HOT</acronym>) updates for

expression indexes when the values of the expressions are unchanged

(Konstantin Knizhnik)

</para>

</listitem>

test: brin gin gist spgist privileges init_privs security_label collate matview lock replica_identity rowsecurity object_address tablesample groupingsets drop_operator password func_index

test: func_index