F.41. spi

F.41. spi
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F.41.1. refint — Functions for Implementing Referential Integrity
F.41.2. autoinc — Functions for Autoincrementing Fields
F.41.3. insert_username — Functions for Tracking Who Changed a Table
F.41.4. moddatetime — Functions for Tracking Last Modification Time

+ The spi module provides several workable examples + of using the Server Programming Interface + (SPI) and triggers. While these functions are of + some value in + their own right, they are even more useful as examples to modify for + your own purposes. The functions are general enough to be used + with any table, but you have to specify table and field names (as described + below) while creating a trigger. +

+ Each of the groups of functions described below is provided as a + separately-installable extension. +

F.41.1. refint — Functions for Implementing Referential Integrity

+ check_primary_key() and + check_foreign_key() are used to check foreign key constraints. + (This functionality is long since superseded by the built-in foreign + key mechanism, of course, but the module is still useful as an example.) +

+ check_primary_key() checks the referencing table. + To use, create a BEFORE INSERT OR UPDATE trigger using this + function on a table referencing another table. Specify as the trigger + arguments: the referencing table's column name(s) which form the foreign + key, the referenced table name, and the column names in the referenced table + which form the primary/unique key. To handle multiple foreign + keys, create a trigger for each reference. +

+ check_foreign_key() checks the referenced table. + To use, create a BEFORE DELETE OR UPDATE trigger using this + function on a table referenced by other table(s). Specify as the trigger + arguments: the number of referencing tables for which the function has to + perform checking, the action if a referencing key is found + (cascade — to delete the referencing row, + restrict — to abort transaction if referencing keys + exist, setnull — to set referencing key fields to null), + the triggered table's column names which form the primary/unique key, then + the referencing table name and column names (repeated for as many + referencing tables as were specified by first argument). Note that the + primary/unique key columns should be marked NOT NULL and should have a + unique index. +

+ There are examples in refint.example. +

F.41.2. autoinc — Functions for Autoincrementing Fields

+ autoinc() is a trigger that stores the next value of + a sequence into an integer field. This has some overlap with the + built-in “serial column” feature, but it is not the same: + autoinc() will override attempts to substitute a + different field value during inserts, and optionally it can be + used to increment the field during updates, too. +

+ To use, create a BEFORE INSERT (or optionally BEFORE + INSERT OR UPDATE) trigger using this function. Specify two + trigger arguments: the name of the integer column to be modified, + and the name of the sequence object that will supply values. + (Actually, you can specify any number of pairs of such names, if + you'd like to update more than one autoincrementing column.) +

+ There is an example in autoinc.example. +

F.41.3. insert_username — Functions for Tracking Who Changed a Table

+ insert_username() is a trigger that stores the current + user's name into a text field. This can be useful for tracking + who last modified a particular row within a table. +

+ To use, create a BEFORE INSERT and/or UPDATE + trigger using this function. Specify a single trigger + argument: the name of the text column to be modified. +

+ There is an example in insert_username.example. +

F.41.4. moddatetime — Functions for Tracking Last Modification Time

+ moddatetime() is a trigger that stores the current + time into a timestamp field. This can be useful for tracking + the last modification time of a particular row within a table. +

+ To use, create a BEFORE UPDATE + trigger using this function. Specify a single trigger + argument: the name of the column to be modified. + The column must be of type timestamp or timestamp with + time zone. +

+ There is an example in moddatetime.example. +

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