|The PostgreSQL 9.0 Reference Manual - Volume 2 - Programming Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 478 pages
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13.4 Visibility of Data Changes
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
INSERT INTO a SELECT * FROM a;the inserted rows are invisible to the
- 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 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.
All standard procedural languages set the SPI read-write mode
depending on the volatility attribute of the function. Commands of
IMMUTABLEfunctions are done in read-only mode, while commands of
VOLATILEfunctions 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.
|ISBN 9781906966065||The PostgreSQL 9.0 Reference Manual - Volume 2 - Programming Guide||See the print edition|