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>43.4. Visibility of Data Changes</A
></H1
><P
> The following rules govern the visibility of data changes in
functions that use SPI (or any other C function):
<P
></P
></P><UL
><LI
><P
> During the execution of an SQL command, any data changes made by
the command are invisible to the command itself. For
example, in:
</P><PRE
CLASS="PROGRAMLISTING"
>INSERT INTO a SELECT * FROM a;</PRE
><P>
the inserted rows are invisible to the <TT
CLASS="COMMAND"
>SELECT</TT
>
part.
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> Changes made by a command C are visible to all commands that are
started after C, no matter whether they are started inside C
(during the execution of C) or after C is done.
</P
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><P
> Commands executed via SPI inside a function called by an SQL command
(either an ordinary function or a trigger) follow one or the
other of the above rules depending on the read/write flag passed
to SPI. Commands executed in read-only mode follow the first
rule: they cannot see changes of the calling command. Commands executed
in read-write mode follow the second rule: they can see all changes made
so far.
</P
></LI
><LI
><P
> All standard procedural languages set the SPI read-write mode
depending on the volatility attribute of the function. Commands of
<TT
CLASS="LITERAL"
>STABLE</TT
> and <TT
CLASS="LITERAL"
>IMMUTABLE</TT
> functions are done in
read-only mode, while commands of <TT
CLASS="LITERAL"
>VOLATILE</TT
> functions are
done in read-write mode. While authors of C functions are able to
violate this convention, it's unlikely to be a good idea to do so.
</P
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> The next section contains an example that illustrates the
application of these rules.
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