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Write a Stored Procedure in Postgres 8+

May 5, 2010 /Ant posted in sql, Technobable / No Comments

Stored Procs

Sometimes as a developer were tasked with data intensive work like importing datainto a database, cleaning up sets of incomplete records or transferring data from onetable to another through some kind offilter. While our application would normally bein charge of creating and maintaining the data, sometimes we dont want to end upwriting an entire module or mini application to address these tasks. Since theyredata intensive, a stored procedure might be a good approach to take. Storedprocedures are a type of program written using a more robust version of sql(structured query language) that allows for the manipulation of data records directlywithin a database environment.

If we were to write the equivalent code using a layer written in java, .net, or php,

there would be a lot of overhead cost in terms of processing power and performance orders of magnitude more. As data is processed, results would normally be returnedto that calling layer and shuffled around that layers memory, essentially addinganother step to the process. If we make these changes as close to the data aspossible, well be able to squeeze as much performance as possible and suffer theleast amount of overhead. Just for perspective heres an example: a 1 gigabyte filecould take several hours to import using java business logic, while a stored proc couldtake less than half an hour. Mileage may vary of course, but thatll give you an idea ofthe performance cost you could save with data intensive tasks like that. A word ofcaution though: Im not saying a stored proc is the way to go for your entireapplication; its merely a tool that can be used in your arsenal to get the job done with

the most efficient means possible.

Example

Heres an example of a generic stored proc written in psql (postgres version).

CREATE OR REPLACEFUNCTION example_stored_proc() RETURNS void AS $$DECLARE

userRecord record;user_property_id bigint;

BEGINFOR userRecord IN

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SELECT * FROM tb_user u ORDERBY u.user_idLOOP

SELECTINTO user_property_id nextval('sq_user_property');

-- user_property_id now has a value we can insert hereINSERTINTO tb_user_property VALUES(

user_property_id ,'user_id',userRecord.id

) ;

IF userRecord.email like 'user@domain.com'THEN

update userRecord set email = 'user@other-domain.com'where id = userRecord.id;

ELSEIF userRecord.email is null THEN

update userRecord set active = falsewhere id = userRecord.id;

ELSE

RAISE NOTICE 'didn't update any record';

END IF;

RAISE NOTICE 'added property foruser id: %', userRecord.id;

END LOOP;RETURN;

END;$$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION example_stored_proc() RETURNS integerAS $$CREATE OR REPLACE FUNCTION will create the stored proc in the database.RETURNS declares the data type returned at the end. This example returns aninteger, but a record or a result set may also be returned. The text in between the twopairs of $$ is the body of the procedure.

DECLAREThis keyword initializes the variables the stored proc will be using. It essentially letsthe database know to allocate memory for use.

BEGINThis marks the beginning of the stored proc logic. It naturally ends with END.

FOR userRecord INSELECT * FROM tb_user u ORDER BY u.user_id

LOOP

- do stuff

END LOOP;

This is the basic looping structure used in psql. Notice the loop is built around astraight forward sql query here is where the magic happens. The looping variable inthis example is userRecord it holds the currently fetched data record and allowsyou to manipulate it for your own means in the body of the loop. So, if you wanted toinsert the value of userRecord.id into a table, you could just stick in the insert

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statement as a variable as shown in the insert statement in particular loops body.

SELECT INTO

Using this construct allows you to create a temporary table to hold query results forlater use. Your variable can be a record or a single column value. In order for it towork you need to declare the variable thats going to take the value in the DECLRAREsection of the stored proc. Inline variable declaration is not supported.

Conditionals

As expected, the IF/THEN/ELSEIF/ELSE/END IF construct can be used to createconditional sequences of logic. The conditionals need to be any kind of expressionpostgres can evaluate. The ELSEIF can be used to wrap secondary conditionals, whilethe ELSE of course is the default if no other conditions are met. Fairly selfexplanatory.

RAISE NOTICE

This is your standard psql logging output statement. The text in the single quotes isoutput to the console/message window, and every % is substituted with the orderedvalue after each comma in the statement. So, in this case userRecord.id issubstituted into the first % to appear in the output text. If you wanted to havemultiple values output you could construct your RAISE NOTICE like this:

RAISE NOTICE 'this is record % out of 1000, and its value is %', record_number, record_value;

It would substitute record_number into the first % and record_value into the second% appearing in the text.

Related posts:

Subverting foreign key constraints in postgres or mysql1.Set up postgres2.Run a huge query as fast and safely as possible3.

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