39.3. Database Access

The PL/Python language module automatically imports a Python module called plpy. The functions and constants in this module are available to you in the Python code as plpy.foo. At present plpy implements the functions plpy.debug(msg), plpy.log(msg), plpy.info(msg), plpy.notice(msg), plpy.warning(msg), plpy.error(msg), and plpy.fatal(msg). plpy.error and plpy.fatal actually raise a Python exception which, if uncaught, propagates out to the calling query, causing the current transaction or subtransaction to be aborted. raise plpy.ERROR(msg) and raise plpy.FATAL(msg) are equivalent to calling plpy.error and plpy.fatal, respectively. The other functions only generate messages of different priority levels. Whether messages of a particular priority are reported to the client, written to the server log, or both is controlled by the log_min_messages and client_min_messages configuration variables. See Chapter 17 for more information.

Additionally, the plpy module provides two functions called execute and prepare. Calling plpy.execute with a query string and an optional limit argument causes that query to be run and the result to be returned in a result object. The result object emulates a list or dictionary object. The result object can be accessed by row number and column name. It has these additional methods: nrows which returns the number of rows returned by the query, and status which is the SPI_execute() return value. The result object can be modified.

For example,

rv = plpy.execute("SELECT * FROM my_table", 5)

returns up to 5 rows from my_table. If my_table has a column my_column, it would be accessed as

foo = rv[i]["my_column"]

The second function, plpy.prepare, prepares the execution plan for a query. It is called with a query string and a list of parameter types, if you have parameter references in the query. For example:

plan = plpy.prepare("SELECT last_name FROM my_users WHERE first_name = $1", [ "text" ])

text is the type of the variable you will be passing for $1. After preparing a statement, you use the function plpy.execute to run it:

rv = plpy.execute(plan, [ "name" ], 5)

The third argument is the limit and is optional.

When you prepare a plan using the PL/Python module it is automatically saved. Read the SPI documentation (Chapter 40) for a description of what this means. In order to make effective use of this across function calls one needs to use one of the persistent storage dictionaries SD or GD (see Section 39.1). For example:

CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
    if SD.has_key("plan"):
        plan = SD["plan"]
    else:
        plan = plpy.prepare("SELECT 1")
        SD["plan"] = plan
    # rest of function
$$ LANGUAGE plpythonu;