The following rules govern the visibility of data changes in
functions that use SPI (or any other C function):
During the execution of an SQL command, any data changes made by
the command are invisible to the command itself. For
example, in
INSERT INTO a SELECT * FROM a;
the inserted rows are invisible to the SELECT
part.
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.
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 can't see changes of the calling command. Commands executed
in read-write mode follow the second rule: they can see all changes made
so far.
All standard procedural languages set the SPI read-write mode
depending on the volatility attribute of the function. Commands of
STABLE and IMMUTABLE functions are done in
read-only mode, while commands of VOLATILE 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.
The next section contains an example that illustrates the
application of these rules.
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