doc:  clarify when expression indexes evaluate their expressions
Only non-HOT updates evaluate the index expression.
Reported-by: Chris Lowder
Discussion:
Backpatch-through: 10
@@ -733,8 +733,8 @@ CREATE INDEX people_names ON people ((first_name || ' ' || last_name));
<para>
Index expressions are relatively expensive to maintain, because the
- derived expression(s) must be computed for each row upon insertion
- and whenever it is updated. However, the index expressions are
+ derived expression(s) must be computed for each row insertion
+ and non-HOT update. However, the index expressions are
<emphasis>not</emphasis> recomputed during an indexed search, since they are
already stored in the index. In both examples above, the system
sees the query as just <literal>WHERE indexedcolumn = 'constant'</literal>
