Contents
Why is the trigger variable special in PostgreSQL?
The new variable in a trigger function is special, representing the row being inserted. Specifying the trigger as a before insert trigger means you can modify the row before it is written to the table.
Can you change the value of a PostgreSQL variable?
If any of the PostgreSQL variables is defined as the CONSTANT, we can not change the variable’s value. We can declare a PostgreSQL variable with a default value, or we can modify it later on as per need if it is not defined CONSTANT.
Why are variables initialized to null in PostgreSQL?
The variable always has a particular data-type give to it like boolean, text, char, integer, double precision, date, time, etc. They are used to store the data which can be changed. The PostgreSQL variables are initialized to the NULL value if they are not defined with DEFAULT value.
How to create a trigger procedure in PostgreSQL?
A trigger procedure is created with the CREATE FUNCTION command, declaring it as a function with no arguments and a return type of trigger. Note that the function must be declared with no arguments even if it expects to receive arguments specified in CREATE TRIGGER — trigger arguments are passed via TG_ARGV, as described below.
How often is the update trigger called in PostgreSQL?
For example: If we UPDATE 100 rows in the table, the UPDATE trigger function will be called 100 times, once for each updated row. 2. Statement Level Trigger: The FOR EACH STATEMENT option will call the trigger function only once for each statement, regardless of the number of the rows getting modified.
Can a constraint trigger be created in PostgreSQL?
Finally, PostgreSQL has the option to create “constraint triggers” with CREATE CONSTRAINT TRIGGER. It sounds like such triggers could be used to avoid the race condition. Constraint triggers respect the MVCC rules, so they cannot “peek” at uncommitted rows of concurrent transactions.
When to return NULL in PostgreSQL trigger function?
Row-level triggers fired BEFORE can return null to signal the trigger manager to skip the rest of the operation for this row (i.e., subsequent triggers are not fired, and the INSERT / UPDATE / DELETE does not occur for this row). If a nonnull value is returned then the operation proceeds with that row value.