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diff --git a/doc/src/sgml/plpython.sgml b/doc/src/sgml/plpython.sgml new file mode 100644 index 0000000..e5d51d6 --- /dev/null +++ b/doc/src/sgml/plpython.sgml @@ -0,0 +1,1397 @@ +<!-- doc/src/sgml/plpython.sgml --> + +<chapter id="plpython"> + <title>PL/Python — Python Procedural Language</title> + + <indexterm zone="plpython"><primary>PL/Python</primary></indexterm> + <indexterm zone="plpython"><primary>Python</primary></indexterm> + + <para> + The <application>PL/Python</application> procedural language allows + <productname>PostgreSQL</productname> functions and procedures to be written in the + <ulink url="https://www.python.org">Python language</ulink>. + </para> + + <para> + To install PL/Python in a particular database, use + <literal>CREATE EXTENSION plpython3u</literal>. + </para> + + <tip> + <para> + If a language is installed into <literal>template1</literal>, all subsequently + created databases will have the language installed automatically. + </para> + </tip> + + <para> + PL/Python is only available as an <quote>untrusted</quote> language, meaning + it does not offer any way of restricting what users can do in it and + is therefore named <literal>plpython3u</literal>. A trusted + variant <literal>plpython</literal> might become available in the future + if a secure execution mechanism is developed in Python. The + writer of a function in untrusted PL/Python must take care that the + function cannot be used to do anything unwanted, since it will be + able to do anything that could be done by a user logged in as the + database administrator. Only superusers can create functions in + untrusted languages such as <literal>plpython3u</literal>. + </para> + + <note> + <para> + Users of source packages must specially enable the build of + PL/Python during the installation process. (Refer to the + installation instructions for more information.) Users of binary + packages might find PL/Python in a separate subpackage. + </para> + </note> + + <sect1 id="plpython-funcs"> + <title>PL/Python Functions</title> + + <para> + Functions in PL/Python are declared via the + standard <xref linkend="sql-createfunction"/> syntax: + +<programlisting> +CREATE FUNCTION <replaceable>funcname</replaceable> (<replaceable>argument-list</replaceable>) + RETURNS <replaceable>return-type</replaceable> +AS $$ + # PL/Python function body +$$ LANGUAGE plpython3u; +</programlisting> + </para> + + <para> + The body of a function is simply a Python script. When the function + is called, its arguments are passed as elements of the list + <varname>args</varname>; named arguments are also passed as + ordinary variables to the Python script. Use of named arguments is + usually more readable. The result is returned from the Python code + in the usual way, with <literal>return</literal> or + <literal>yield</literal> (in case of a result-set statement). If + you do not provide a return value, Python returns the default + <symbol>None</symbol>. <application>PL/Python</application> translates + Python's <symbol>None</symbol> into the SQL null value. In a procedure, + the result from the Python code must be <symbol>None</symbol> (typically + achieved by ending the procedure without a <literal>return</literal> + statement or by using a <literal>return</literal> statement without + argument); otherwise, an error will be raised. + </para> + + <para> + For example, a function to return the greater of two integers can be + defined as: + +<programlisting> +CREATE FUNCTION pymax (a integer, b integer) + RETURNS integer +AS $$ + if a > b: + return a + return b +$$ LANGUAGE plpython3u; +</programlisting> + + The Python code that is given as the body of the function definition + is transformed into a Python function. For example, the above results in: + +<programlisting> +def __plpython_procedure_pymax_23456(): + if a > b: + return a + return b +</programlisting> + + assuming that 23456 is the OID assigned to the function by + <productname>PostgreSQL</productname>. + </para> + + <para> + The arguments are set as global variables. Because of the scoping + rules of Python, this has the subtle consequence that an argument + variable cannot be reassigned inside the function to the value of + an expression that involves the variable name itself, unless the + variable is redeclared as global in the block. For example, the + following won't work: +<programlisting> +CREATE FUNCTION pystrip(x text) + RETURNS text +AS $$ + x = x.strip() # error + return x +$$ LANGUAGE plpython3u; +</programlisting> + because assigning to <varname>x</varname> + makes <varname>x</varname> a local variable for the entire block, + and so the <varname>x</varname> on the right-hand side of the + assignment refers to a not-yet-assigned local + variable <varname>x</varname>, not the PL/Python function + parameter. Using the <literal>global</literal> statement, this can + be made to work: +<programlisting> +CREATE FUNCTION pystrip(x text) + RETURNS text +AS $$ + global x + x = x.strip() # ok now + return x +$$ LANGUAGE plpython3u; +</programlisting> + But it is advisable not to rely on this implementation detail of + PL/Python. It is better to treat the function parameters as + read-only. + </para> + </sect1> + + <sect1 id="plpython-data"> + <title>Data Values</title> + <para> + Generally speaking, the aim of PL/Python is to provide + a <quote>natural</quote> mapping between the PostgreSQL and the + Python worlds. This informs the data mapping rules described + below. + </para> + + <sect2 id="plpython-data-type-mapping"> + <title>Data Type Mapping</title> + <para> + When a PL/Python function is called, its arguments are converted from + their PostgreSQL data type to a corresponding Python type: + + <itemizedlist> + <listitem> + <para> + PostgreSQL <type>boolean</type> is converted to Python <type>bool</type>. + </para> + </listitem> + + <listitem> + <para> + PostgreSQL <type>smallint</type>, <type>int</type>, <type>bigint</type> + and <type>oid</type> are converted to Python <type>int</type>. + </para> + </listitem> + + <listitem> + <para> + PostgreSQL <type>real</type> and <type>double</type> are converted to + Python <type>float</type>. + </para> + </listitem> + + <listitem> + <para> + PostgreSQL <type>numeric</type> is converted to + Python <type>Decimal</type>. This type is imported from + the <literal>cdecimal</literal> package if that is available. + Otherwise, + <literal>decimal.Decimal</literal> from the standard library will be + used. <literal>cdecimal</literal> is significantly faster + than <literal>decimal</literal>. In Python 3.3 and up, + however, <literal>cdecimal</literal> has been integrated into the + standard library under the name <literal>decimal</literal>, so there is + no longer any difference. + </para> + </listitem> + + <listitem> + <para> + PostgreSQL <type>bytea</type> is converted to Python <type>bytes</type>. + </para> + </listitem> + + <listitem> + <para> + All other data types, including the PostgreSQL character string types, + are converted to a Python <type>str</type> (in Unicode like all Python + strings). + </para> + </listitem> + + <listitem> + <para> + For nonscalar data types, see below. + </para> + </listitem> + </itemizedlist> + </para> + + <para> + When a PL/Python function returns, its return value is converted to the + function's declared PostgreSQL return data type as follows: + + <itemizedlist> + <listitem> + <para> + When the PostgreSQL return type is <type>boolean</type>, the + return value will be evaluated for truth according to the + <emphasis>Python</emphasis> rules. That is, 0 and empty string + are false, but notably <literal>'f'</literal> is true. + </para> + </listitem> + + <listitem> + <para> + When the PostgreSQL return type is <type>bytea</type>, the return value + will be converted to Python <type>bytes</type> using the respective + Python built-ins, with the result being converted to + <type>bytea</type>. + </para> + </listitem> + + <listitem> + <para> + For all other PostgreSQL return types, the return value is converted + to a string using the Python built-in <literal>str</literal>, and the + result is passed to the input function of the PostgreSQL data type. + (If the Python value is a <type>float</type>, it is converted using + the <literal>repr</literal> built-in instead of <literal>str</literal>, to + avoid loss of precision.) + </para> + + <para> + Strings are automatically converted to the PostgreSQL server encoding + when they are passed to PostgreSQL. + </para> + </listitem> + + <listitem> + <para> + For nonscalar data types, see below. + </para> + </listitem> + </itemizedlist> + + Note that logical mismatches between the declared PostgreSQL + return type and the Python data type of the actual return object + are not flagged; the value will be converted in any case. + </para> + </sect2> + + <sect2 id="plpython-data-null"> + <title>Null, None</title> + <para> + If an SQL null value<indexterm><primary>null value</primary><secondary + sortas="PL/Python">in PL/Python</secondary></indexterm> is passed to a + function, the argument value will appear as <symbol>None</symbol> in + Python. For example, the function definition of <function>pymax</function> + shown in <xref linkend="plpython-funcs"/> will return the wrong answer for null + inputs. We could add <literal>STRICT</literal> to the function definition + to make <productname>PostgreSQL</productname> do something more reasonable: + if a null value is passed, the function will not be called at all, + but will just return a null result automatically. Alternatively, + we could check for null inputs in the function body: + +<programlisting> +CREATE FUNCTION pymax (a integer, b integer) + RETURNS integer +AS $$ + if (a is None) or (b is None): + return None + if a > b: + return a + return b +$$ LANGUAGE plpython3u; +</programlisting> + + As shown above, to return an SQL null value from a PL/Python + function, return the value <symbol>None</symbol>. This can be done whether the + function is strict or not. + </para> + </sect2> + + <sect2 id="plpython-arrays"> + <title>Arrays, Lists</title> + <para> + SQL array values are passed into PL/Python as a Python list. To + return an SQL array value out of a PL/Python function, return a + Python list: + +<programlisting> +CREATE FUNCTION return_arr() + RETURNS int[] +AS $$ +return [1, 2, 3, 4, 5] +$$ LANGUAGE plpython3u; + +SELECT return_arr(); + return_arr +------------- + {1,2,3,4,5} +(1 row) +</programlisting> + + Multidimensional arrays are passed into PL/Python as nested Python lists. + A 2-dimensional array is a list of lists, for example. When returning + a multi-dimensional SQL array out of a PL/Python function, the inner + lists at each level must all be of the same size. For example: + +<programlisting> +CREATE FUNCTION test_type_conversion_array_int4(x int4[]) RETURNS int4[] AS $$ +plpy.info(x, type(x)) +return x +$$ LANGUAGE plpython3u; + +SELECT * FROM test_type_conversion_array_int4(ARRAY[[1,2,3],[4,5,6]]); +INFO: ([[1, 2, 3], [4, 5, 6]], <type 'list'>) + test_type_conversion_array_int4 +--------------------------------- + {{1,2,3},{4,5,6}} +(1 row) +</programlisting> + + Other Python sequences, like tuples, are also accepted for + backwards-compatibility with PostgreSQL versions 9.6 and below, when + multi-dimensional arrays were not supported. However, they are always + treated as one-dimensional arrays, because they are ambiguous with + composite types. For the same reason, when a composite type is used in a + multi-dimensional array, it must be represented by a tuple, rather than a + list. + </para> + <para> + Note that in Python, strings are sequences, which can have + undesirable effects that might be familiar to Python programmers: + +<programlisting> +CREATE FUNCTION return_str_arr() + RETURNS varchar[] +AS $$ +return "hello" +$$ LANGUAGE plpython3u; + +SELECT return_str_arr(); + return_str_arr +---------------- + {h,e,l,l,o} +(1 row) +</programlisting> + </para> + </sect2> + + <sect2 id="plpython-data-composite-types"> + <title>Composite Types</title> + <para> + Composite-type arguments are passed to the function as Python mappings. The + element names of the mapping are the attribute names of the composite type. + If an attribute in the passed row has the null value, it has the value + <symbol>None</symbol> in the mapping. Here is an example: + +<programlisting> +CREATE TABLE employee ( + name text, + salary integer, + age integer +); + +CREATE FUNCTION overpaid (e employee) + RETURNS boolean +AS $$ + if e["salary"] > 200000: + return True + if (e["age"] < 30) and (e["salary"] > 100000): + return True + return False +$$ LANGUAGE plpython3u; +</programlisting> + </para> + + <para> + There are multiple ways to return row or composite types from a Python + function. The following examples assume we have: + +<programlisting> +CREATE TYPE named_value AS ( + name text, + value integer +); +</programlisting> + + A composite result can be returned as a: + + <variablelist> + <varlistentry> + <term>Sequence type (a tuple or list, but not a set because + it is not indexable)</term> + <listitem> + <para> + Returned sequence objects must have the same number of items as the + composite result type has fields. The item with index 0 is assigned to + the first field of the composite type, 1 to the second and so on. For + example: + +<programlisting> +CREATE FUNCTION make_pair (name text, value integer) + RETURNS named_value +AS $$ + return ( name, value ) + # or alternatively, as list: return [ name, value ] +$$ LANGUAGE plpython3u; +</programlisting> + + To return an SQL null for any column, insert <symbol>None</symbol> at + the corresponding position. + </para> + <para> + When an array of composite types is returned, it cannot be returned as a list, + because it is ambiguous whether the Python list represents a composite type, + or another array dimension. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Mapping (dictionary)</term> + <listitem> + <para> + The value for each result type column is retrieved from the mapping + with the column name as key. Example: + +<programlisting> +CREATE FUNCTION make_pair (name text, value integer) + RETURNS named_value +AS $$ + return { "name": name, "value": value } +$$ LANGUAGE plpython3u; +</programlisting> + + Any extra dictionary key/value pairs are ignored. Missing keys are + treated as errors. + To return an SQL null value for any column, insert + <symbol>None</symbol> with the corresponding column name as the key. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Object (any object providing method <literal>__getattr__</literal>)</term> + <listitem> + <para> + This works the same as a mapping. + Example: + +<programlisting> +CREATE FUNCTION make_pair (name text, value integer) + RETURNS named_value +AS $$ + class named_value: + def __init__ (self, n, v): + self.name = n + self.value = v + return named_value(name, value) + + # or simply + class nv: pass + nv.name = name + nv.value = value + return nv +$$ LANGUAGE plpython3u; +</programlisting> + </para> + </listitem> + </varlistentry> + </variablelist> + </para> + + <para> + Functions with <literal>OUT</literal> parameters are also supported. For example: +<programlisting> +CREATE FUNCTION multiout_simple(OUT i integer, OUT j integer) AS $$ +return (1, 2) +$$ LANGUAGE plpython3u; + +SELECT * FROM multiout_simple(); +</programlisting> + </para> + + <para> + Output parameters of procedures are passed back the same way. For example: +<programlisting> +CREATE PROCEDURE python_triple(INOUT a integer, INOUT b integer) AS $$ +return (a * 3, b * 3) +$$ LANGUAGE plpython3u; + +CALL python_triple(5, 10); +</programlisting> + </para> + </sect2> + + <sect2 id="plpython-data-set-returning-funcs"> + <title>Set-Returning Functions</title> + <para> + A <application>PL/Python</application> function can also return sets of + scalar or composite types. There are several ways to achieve this because + the returned object is internally turned into an iterator. The following + examples assume we have composite type: + +<programlisting> +CREATE TYPE greeting AS ( + how text, + who text +); +</programlisting> + + A set result can be returned from a: + + <variablelist> + <varlistentry> + <term>Sequence type (tuple, list, set)</term> + <listitem> + <para> +<programlisting> +CREATE FUNCTION greet (how text) + RETURNS SETOF greeting +AS $$ + # return tuple containing lists as composite types + # all other combinations work also + return ( [ how, "World" ], [ how, "PostgreSQL" ], [ how, "PL/Python" ] ) +$$ LANGUAGE plpython3u; +</programlisting> + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Iterator (any object providing <symbol>__iter__</symbol> and + <symbol>next</symbol> methods)</term> + <listitem> + <para> +<programlisting> +CREATE FUNCTION greet (how text) + RETURNS SETOF greeting +AS $$ + class producer: + def __init__ (self, how, who): + self.how = how + self.who = who + self.ndx = -1 + + def __iter__ (self): + return self + + def next (self): + self.ndx += 1 + if self.ndx == len(self.who): + raise StopIteration + return ( self.how, self.who[self.ndx] ) + + return producer(how, [ "World", "PostgreSQL", "PL/Python" ]) +$$ LANGUAGE plpython3u; +</programlisting> + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Generator (<literal>yield</literal>)</term> + <listitem> + <para> +<programlisting> +CREATE FUNCTION greet (how text) + RETURNS SETOF greeting +AS $$ + for who in [ "World", "PostgreSQL", "PL/Python" ]: + yield ( how, who ) +$$ LANGUAGE plpython3u; +</programlisting> + + </para> + </listitem> + </varlistentry> + </variablelist> + </para> + + <para> + Set-returning functions with <literal>OUT</literal> parameters + (using <literal>RETURNS SETOF record</literal>) are also + supported. For example: +<programlisting> +CREATE FUNCTION multiout_simple_setof(n integer, OUT integer, OUT integer) RETURNS SETOF record AS $$ +return [(1, 2)] * n +$$ LANGUAGE plpython3u; + +SELECT * FROM multiout_simple_setof(3); +</programlisting> + </para> + </sect2> + </sect1> + + <sect1 id="plpython-sharing"> + <title>Sharing Data</title> + <para> + The global dictionary <varname>SD</varname> is available to store + private data between repeated calls to the same function. + The global dictionary <varname>GD</varname> is public data, + that is available to all Python functions within a session; use with + care.<indexterm><primary>global data</primary> + <secondary>in PL/Python</secondary></indexterm> + </para> + + <para> + Each function gets its own execution environment in the + Python interpreter, so that global data and function arguments from + <function>myfunc</function> are not available to + <function>myfunc2</function>. The exception is the data in the + <varname>GD</varname> dictionary, as mentioned above. + </para> + </sect1> + + <sect1 id="plpython-do"> + <title>Anonymous Code Blocks</title> + + <para> + PL/Python also supports anonymous code blocks called with the + <xref linkend="sql-do"/> statement: + +<programlisting> +DO $$ + # PL/Python code +$$ LANGUAGE plpython3u; +</programlisting> + + An anonymous code block receives no arguments, and whatever value it + might return is discarded. Otherwise it behaves just like a function. + </para> + </sect1> + + <sect1 id="plpython-trigger"> + <title>Trigger Functions</title> + + <indexterm zone="plpython-trigger"> + <primary>trigger</primary> + <secondary>in PL/Python</secondary> + </indexterm> + + <para> + When a function is used as a trigger, the dictionary + <literal>TD</literal> contains trigger-related values: + <variablelist> + <varlistentry> + <term><literal>TD["event"]</literal></term> + <listitem> + <para> + contains the event as a string: + <literal>INSERT</literal>, <literal>UPDATE</literal>, + <literal>DELETE</literal>, or <literal>TRUNCATE</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>TD["when"]</literal></term> + <listitem> + <para> + contains one of <literal>BEFORE</literal>, <literal>AFTER</literal>, or + <literal>INSTEAD OF</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>TD["level"]</literal></term> + <listitem> + <para> + contains <literal>ROW</literal> or <literal>STATEMENT</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>TD["new"]</literal></term> + <term><literal>TD["old"]</literal></term> + <listitem> + <para> + For a row-level trigger, one or both of these fields contain + the respective trigger rows, depending on the trigger event. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>TD["name"]</literal></term> + <listitem> + <para> + contains the trigger name. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>TD["table_name"]</literal></term> + <listitem> + <para> + contains the name of the table on which the trigger occurred. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>TD["table_schema"]</literal></term> + <listitem> + <para> + contains the schema of the table on which the trigger occurred. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>TD["relid"]</literal></term> + <listitem> + <para> + contains the OID of the table on which the trigger occurred. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>TD["args"]</literal></term> + <listitem> + <para> + If the <command>CREATE TRIGGER</command> command + included arguments, they are available in <literal>TD["args"][0]</literal> to + <literal>TD["args"][<replaceable>n</replaceable>-1]</literal>. + </para> + </listitem> + </varlistentry> + </variablelist> + </para> + + <para> + If <literal>TD["when"]</literal> is <literal>BEFORE</literal> or + <literal>INSTEAD OF</literal> and + <literal>TD["level"]</literal> is <literal>ROW</literal>, you can + return <literal>None</literal> or <literal>"OK"</literal> from the + Python function to indicate the row is unmodified, + <literal>"SKIP"</literal> to abort the event, or if <literal>TD["event"]</literal> + is <command>INSERT</command> or <command>UPDATE</command> you can return + <literal>"MODIFY"</literal> to indicate you've modified the new row. + Otherwise the return value is ignored. + </para> + </sect1> + + <sect1 id="plpython-database"> + <title>Database Access</title> + + <para> + The PL/Python language module automatically imports a Python module + called <literal>plpy</literal>. The functions and constants in + this module are available to you in the Python code as + <literal>plpy.<replaceable>foo</replaceable></literal>. + </para> + + <sect2 id="plpython-database-access-funcs"> + <title>Database Access Functions</title> + + <para> + The <literal>plpy</literal> module provides several functions to execute + database commands: + </para> + + <variablelist> + <varlistentry> + <term><literal>plpy.<function>execute</function>(<replaceable>query</replaceable> [, <replaceable>limit</replaceable>])</literal></term> + <listitem> + <para> + Calling <function>plpy.execute</function> with a query string and an + optional row limit argument causes that query to be run and the result to + be returned in a result object. + </para> + + <para> + If <replaceable>limit</replaceable> is specified and is greater than + zero, then <function>plpy.execute</function> retrieves at + most <replaceable>limit</replaceable> rows, much as if the query + included a <literal>LIMIT</literal> + clause. Omitting <replaceable>limit</replaceable> or specifying it as + zero results in no row limit. + </para> + + <para> + The result object emulates a list or dictionary object. The result + object can be accessed by row number and column name. For example: +<programlisting> +rv = plpy.execute("SELECT * FROM my_table", 5) +</programlisting> + returns up to 5 rows from <literal>my_table</literal>. If + <literal>my_table</literal> has a column + <literal>my_column</literal>, it would be accessed as: +<programlisting> +foo = rv[i]["my_column"] +</programlisting> + The number of rows returned can be obtained using the built-in + <function>len</function> function. + </para> + + <para> + The result object has these additional methods: + <variablelist> + <varlistentry> + <term><literal><function>nrows</function>()</literal></term> + <listitem> + <para> + Returns the number of rows processed by the command. Note that this + is not necessarily the same as the number of rows returned. For + example, an <command>UPDATE</command> command will set this value but + won't return any rows (unless <literal>RETURNING</literal> is used). + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal><function>status</function>()</literal></term> + <listitem> + <para> + The <function>SPI_execute()</function> return value. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal><function>colnames</function>()</literal></term> + <term><literal><function>coltypes</function>()</literal></term> + <term><literal><function>coltypmods</function>()</literal></term> + <listitem> + <para> + Return a list of column names, list of column type OIDs, and list of + type-specific type modifiers for the columns, respectively. + </para> + + <para> + These methods raise an exception when called on a result object from + a command that did not produce a result set, e.g., + <command>UPDATE</command> without <literal>RETURNING</literal>, or + <command>DROP TABLE</command>. But it is OK to use these methods on + a result set containing zero rows. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal><function>__str__</function>()</literal></term> + <listitem> + <para> + The standard <literal>__str__</literal> method is defined so that it + is possible for example to debug query execution results + using <literal>plpy.debug(rv)</literal>. + </para> + </listitem> + </varlistentry> + </variablelist> + </para> + + <para> + The result object can be modified. + </para> + + <para> + Note that calling <literal>plpy.execute</literal> will cause the entire + result set to be read into memory. Only use that function when you are + sure that the result set will be relatively small. If you don't want to + risk excessive memory usage when fetching large results, + use <literal>plpy.cursor</literal> rather + than <literal>plpy.execute</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>plpy.<function>prepare</function>(<replaceable>query</replaceable> [, <replaceable>argtypes</replaceable>])</literal></term> + <term><literal>plpy.<function>execute</function>(<replaceable>plan</replaceable> [, <replaceable>arguments</replaceable> [, <replaceable>limit</replaceable>]])</literal></term> + <listitem> + <para> + <indexterm><primary>preparing a query</primary><secondary>in PL/Python</secondary></indexterm> + <function>plpy.prepare</function> 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: +<programlisting> +plan = plpy.prepare("SELECT last_name FROM my_users WHERE first_name = $1", ["text"]) +</programlisting> + <literal>text</literal> is the type of the variable you will be passing + for <literal>$1</literal>. The second argument is optional if you don't + want to pass any parameters to the query. + </para> + <para> + After preparing a statement, you use a variant of the + function <function>plpy.execute</function> to run it: +<programlisting> +rv = plpy.execute(plan, ["name"], 5) +</programlisting> + Pass the plan as the first argument (instead of the query string), and a + list of values to substitute into the query as the second argument. The + second argument is optional if the query does not expect any parameters. + The third argument is the optional row limit as before. + </para> + + <para> + Alternatively, you can call the <function>execute</function> method on + the plan object: +<programlisting> +rv = plan.execute(["name"], 5) +</programlisting> + </para> + + <para> + Query parameters and result row fields are converted between PostgreSQL + and Python data types as described in <xref linkend="plpython-data"/>. + </para> + + <para> + When you prepare a plan using the PL/Python module it is automatically + saved. Read the SPI documentation (<xref linkend="spi"/>) 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 <literal>SD</literal> or <literal>GD</literal> (see + <xref linkend="plpython-sharing"/>). For example: +<programlisting> +CREATE FUNCTION usesavedplan() RETURNS trigger AS $$ + if "plan" in SD: + plan = SD["plan"] + else: + plan = plpy.prepare("SELECT 1") + SD["plan"] = plan + # rest of function +$$ LANGUAGE plpython3u; +</programlisting> + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>plpy.<function>cursor</function>(<replaceable>query</replaceable>)</literal></term> + <term><literal>plpy.<function>cursor</function>(<replaceable>plan</replaceable> [, <replaceable>arguments</replaceable>])</literal></term> + <listitem> + <para> + The <literal>plpy.cursor</literal> function accepts the same arguments + as <literal>plpy.execute</literal> (except for the row limit) and returns + a cursor object, which allows you to process large result sets in smaller + chunks. As with <literal>plpy.execute</literal>, either a query string + or a plan object along with a list of arguments can be used, or + the <function>cursor</function> function can be called as a method of + the plan object. + </para> + + <para> + The cursor object provides a <literal>fetch</literal> method that accepts + an integer parameter and returns a result object. Each time you + call <literal>fetch</literal>, the returned object will contain the next + batch of rows, never larger than the parameter value. Once all rows are + exhausted, <literal>fetch</literal> starts returning an empty result + object. Cursor objects also provide an + <ulink url="https://docs.python.org/library/stdtypes.html#iterator-types">iterator + interface</ulink>, yielding one row at a time until all rows are + exhausted. Data fetched that way is not returned as result objects, but + rather as dictionaries, each dictionary corresponding to a single result + row. + </para> + + <para> + An example of two ways of processing data from a large table is: +<programlisting> +CREATE FUNCTION count_odd_iterator() RETURNS integer AS $$ +odd = 0 +for row in plpy.cursor("select num from largetable"): + if row['num'] % 2: + odd += 1 +return odd +$$ LANGUAGE plpython3u; + +CREATE FUNCTION count_odd_fetch(batch_size integer) RETURNS integer AS $$ +odd = 0 +cursor = plpy.cursor("select num from largetable") +while True: + rows = cursor.fetch(batch_size) + if not rows: + break + for row in rows: + if row['num'] % 2: + odd += 1 +return odd +$$ LANGUAGE plpython3u; + +CREATE FUNCTION count_odd_prepared() RETURNS integer AS $$ +odd = 0 +plan = plpy.prepare("select num from largetable where num % $1 <> 0", ["integer"]) +rows = list(plpy.cursor(plan, [2])) # or: = list(plan.cursor([2])) + +return len(rows) +$$ LANGUAGE plpython3u; +</programlisting> + </para> + + <para> + Cursors are automatically disposed of. But if you want to explicitly + release all resources held by a cursor, use the <literal>close</literal> + method. Once closed, a cursor cannot be fetched from anymore. + </para> + + <tip> + <para> + Do not confuse objects created by <literal>plpy.cursor</literal> with + DB-API cursors as defined by + the <ulink url="https://www.python.org/dev/peps/pep-0249/">Python + Database API specification</ulink>. They don't have anything in common + except for the name. + </para> + </tip> + </listitem> + </varlistentry> + </variablelist> + + </sect2> + + <sect2 id="plpython-trapping"> + <title>Trapping Errors</title> + + <para> + Functions accessing the database might encounter errors, which + will cause them to abort and raise an exception. Both + <function>plpy.execute</function> and + <function>plpy.prepare</function> can raise an instance of a subclass of + <literal>plpy.SPIError</literal>, which by default will terminate + the function. This error can be handled just like any other + Python exception, by using the <literal>try/except</literal> + construct. For example: +<programlisting> +CREATE FUNCTION try_adding_joe() RETURNS text AS $$ + try: + plpy.execute("INSERT INTO users(username) VALUES ('joe')") + except plpy.SPIError: + return "something went wrong" + else: + return "Joe added" +$$ LANGUAGE plpython3u; +</programlisting> + </para> + + <para> + The actual class of the exception being raised corresponds to the + specific condition that caused the error. Refer + to <xref linkend="errcodes-table"/> for a list of possible + conditions. The module + <literal>plpy.spiexceptions</literal> defines an exception class + for each <productname>PostgreSQL</productname> condition, deriving + their names from the condition name. For + instance, <literal>division_by_zero</literal> + becomes <literal>DivisionByZero</literal>, <literal>unique_violation</literal> + becomes <literal>UniqueViolation</literal>, <literal>fdw_error</literal> + becomes <literal>FdwError</literal>, and so on. Each of these + exception classes inherits from <literal>SPIError</literal>. This + separation makes it easier to handle specific errors, for + instance: +<programlisting> +CREATE FUNCTION insert_fraction(numerator int, denominator int) RETURNS text AS $$ +from plpy import spiexceptions +try: + plan = plpy.prepare("INSERT INTO fractions (frac) VALUES ($1 / $2)", ["int", "int"]) + plpy.execute(plan, [numerator, denominator]) +except spiexceptions.DivisionByZero: + return "denominator cannot equal zero" +except spiexceptions.UniqueViolation: + return "already have that fraction" +except plpy.SPIError as e: + return "other error, SQLSTATE %s" % e.sqlstate +else: + return "fraction inserted" +$$ LANGUAGE plpython3u; +</programlisting> + Note that because all exceptions from + the <literal>plpy.spiexceptions</literal> module inherit + from <literal>SPIError</literal>, an <literal>except</literal> + clause handling it will catch any database access error. + </para> + + <para> + As an alternative way of handling different error conditions, you + can catch the <literal>SPIError</literal> exception and determine + the specific error condition inside the <literal>except</literal> + block by looking at the <literal>sqlstate</literal> attribute of + the exception object. This attribute is a string value containing + the <quote>SQLSTATE</quote> error code. This approach provides + approximately the same functionality + </para> + </sect2> + </sect1> + + <sect1 id="plpython-subtransaction"> + <title>Explicit Subtransactions</title> + + <para> + Recovering from errors caused by database access as described in + <xref linkend="plpython-trapping"/> can lead to an undesirable + situation where some operations succeed before one of them fails, + and after recovering from that error the data is left in an + inconsistent state. PL/Python offers a solution to this problem in + the form of explicit subtransactions. + </para> + + <sect2 id="plpython-subtransaction-context-managers"> + <title>Subtransaction Context Managers</title> + + <para> + Consider a function that implements a transfer between two + accounts: +<programlisting> +CREATE FUNCTION transfer_funds() RETURNS void AS $$ +try: + plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'") + plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'") +except plpy.SPIError as e: + result = "error transferring funds: %s" % e.args +else: + result = "funds transferred correctly" +plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"]) +plpy.execute(plan, [result]) +$$ LANGUAGE plpython3u; +</programlisting> + If the second <literal>UPDATE</literal> statement results in an + exception being raised, this function will report the error, but + the result of the first <literal>UPDATE</literal> will + nevertheless be committed. In other words, the funds will be + withdrawn from Joe's account, but will not be transferred to + Mary's account. + </para> + + <para> + To avoid such issues, you can wrap your + <literal>plpy.execute</literal> calls in an explicit + subtransaction. The <literal>plpy</literal> module provides a + helper object to manage explicit subtransactions that gets created + with the <literal>plpy.subtransaction()</literal> function. + Objects created by this function implement the + <ulink url="https://docs.python.org/library/stdtypes.html#context-manager-types"> + context manager interface</ulink>. Using explicit subtransactions + we can rewrite our function as: +<programlisting> +CREATE FUNCTION transfer_funds2() RETURNS void AS $$ +try: + with plpy.subtransaction(): + plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'") + plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'") +except plpy.SPIError as e: + result = "error transferring funds: %s" % e.args +else: + result = "funds transferred correctly" +plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"]) +plpy.execute(plan, [result]) +$$ LANGUAGE plpython3u; +</programlisting> + Note that the use of <literal>try</literal>/<literal>except</literal> is still + required. Otherwise the exception would propagate to the top of + the Python stack and would cause the whole function to abort with + a <productname>PostgreSQL</productname> error, so that the + <literal>operations</literal> table would not have any row + inserted into it. The subtransaction context manager does not + trap errors, it only assures that all database operations executed + inside its scope will be atomically committed or rolled back. A + rollback of the subtransaction block occurs on any kind of + exception exit, not only ones caused by errors originating from + database access. A regular Python exception raised inside an + explicit subtransaction block would also cause the subtransaction + to be rolled back. + </para> + </sect2> + </sect1> + + <sect1 id="plpython-transactions"> + <title>Transaction Management</title> + + <para> + In a procedure called from the top level or an anonymous code block + (<command>DO</command> command) called from the top level it is possible to + control transactions. To commit the current transaction, call + <literal>plpy.commit()</literal>. To roll back the current transaction, + call <literal>plpy.rollback()</literal>. (Note that it is not possible to + run the SQL commands <command>COMMIT</command> or + <command>ROLLBACK</command> via <function>plpy.execute</function> or + similar. It has to be done using these functions.) After a transaction is + ended, a new transaction is automatically started, so there is no separate + function for that. + </para> + + <para> + Here is an example: +<programlisting> +CREATE PROCEDURE transaction_test1() +LANGUAGE plpython3u +AS $$ +for i in range(0, 10): + plpy.execute("INSERT INTO test1 (a) VALUES (%d)" % i) + if i % 2 == 0: + plpy.commit() + else: + plpy.rollback() +$$; + +CALL transaction_test1(); +</programlisting> + </para> + + <para> + Transactions cannot be ended when an explicit subtransaction is active. + </para> + </sect1> + + <sect1 id="plpython-util"> + <title>Utility Functions</title> + <para> + The <literal>plpy</literal> module also provides the functions + <simplelist> + <member><literal>plpy.debug(<replaceable>msg, **kwargs</replaceable>)</literal></member> + <member><literal>plpy.log(<replaceable>msg, **kwargs</replaceable>)</literal></member> + <member><literal>plpy.info(<replaceable>msg, **kwargs</replaceable>)</literal></member> + <member><literal>plpy.notice(<replaceable>msg, **kwargs</replaceable>)</literal></member> + <member><literal>plpy.warning(<replaceable>msg, **kwargs</replaceable>)</literal></member> + <member><literal>plpy.error(<replaceable>msg, **kwargs</replaceable>)</literal></member> + <member><literal>plpy.fatal(<replaceable>msg, **kwargs</replaceable>)</literal></member> + </simplelist> + <indexterm><primary>elog</primary><secondary>in PL/Python</secondary></indexterm> + <function>plpy.error</function> and <function>plpy.fatal</function> + actually raise a Python exception which, if uncaught, propagates out to + the calling query, causing the current transaction or subtransaction to + be aborted. <literal>raise plpy.Error(<replaceable>msg</replaceable>)</literal> and + <literal>raise plpy.Fatal(<replaceable>msg</replaceable>)</literal> are + equivalent to calling <literal>plpy.error(<replaceable>msg</replaceable>)</literal> and + <literal>plpy.fatal(<replaceable>msg</replaceable>)</literal>, respectively but + the <literal>raise</literal> form does not allow passing keyword arguments. + 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 + <xref linkend="guc-log-min-messages"/> and + <xref linkend="guc-client-min-messages"/> configuration + variables. See <xref linkend="runtime-config"/> for more information. + </para> + + <para> + The <replaceable>msg</replaceable> argument is given as a positional argument. For + backward compatibility, more than one positional argument can be given. In + that case, the string representation of the tuple of positional arguments + becomes the message reported to the client. + </para> + + <para> + The following keyword-only arguments are accepted: + <simplelist> + <member><literal>detail</literal></member> + <member><literal>hint</literal></member> + <member><literal>sqlstate</literal></member> + <member><literal>schema_name</literal></member> + <member><literal>table_name</literal></member> + <member><literal>column_name</literal></member> + <member><literal>datatype_name</literal></member> + <member><literal>constraint_name</literal></member> + </simplelist> + The string representation of the objects passed as keyword-only arguments + is used to enrich the messages reported to the client. For example: + +<programlisting> +CREATE FUNCTION raise_custom_exception() RETURNS void AS $$ +plpy.error("custom exception message", + detail="some info about exception", + hint="hint for users") +$$ LANGUAGE plpython3u; + +=# SELECT raise_custom_exception(); +ERROR: plpy.Error: custom exception message +DETAIL: some info about exception +HINT: hint for users +CONTEXT: Traceback (most recent call last): + PL/Python function "raise_custom_exception", line 4, in <module> + hint="hint for users") +PL/Python function "raise_custom_exception" +</programlisting> + </para> + + <para> + Another set of utility functions are + <literal>plpy.quote_literal(<replaceable>string</replaceable>)</literal>, + <literal>plpy.quote_nullable(<replaceable>string</replaceable>)</literal>, and + <literal>plpy.quote_ident(<replaceable>string</replaceable>)</literal>. They + are equivalent to the built-in quoting functions described in <xref + linkend="functions-string"/>. They are useful when constructing + ad-hoc queries. A PL/Python equivalent of dynamic SQL from <xref + linkend="plpgsql-quote-literal-example"/> would be: +<programlisting> +plpy.execute("UPDATE tbl SET %s = %s WHERE key = %s" % ( + plpy.quote_ident(colname), + plpy.quote_nullable(newvalue), + plpy.quote_literal(keyvalue))) +</programlisting> + </para> + </sect1> + + <sect1 id="plpython-python23"> + <title>Python 2 vs. Python 3</title> + + <para> + PL/Python supports only Python 3. Past versions of + <productname>PostgreSQL</productname> supported Python 2, using the + <literal>plpythonu</literal> and <literal>plpython2u</literal> language + names. + </para> + </sect1> + + <sect1 id="plpython-envar"> + <title>Environment Variables</title> + + <para> + Some of the environment variables that are accepted by the Python + interpreter can also be used to affect PL/Python behavior. They + would need to be set in the environment of the main PostgreSQL + server process, for example in a start script. The available + environment variables depend on the version of Python; see the + Python documentation for details. At the time of this writing, the + following environment variables have an affect on PL/Python, + assuming an adequate Python version: + <itemizedlist> + <listitem> + <para><envar>PYTHONHOME</envar></para> + </listitem> + + <listitem> + <para><envar>PYTHONPATH</envar></para> + </listitem> + + <listitem> + <para><envar>PYTHONY2K</envar></para> + </listitem> + + <listitem> + <para><envar>PYTHONOPTIMIZE</envar></para> + </listitem> + + <listitem> + <para><envar>PYTHONDEBUG</envar></para> + </listitem> + + <listitem> + <para><envar>PYTHONVERBOSE</envar></para> + </listitem> + + <listitem> + <para><envar>PYTHONCASEOK</envar></para> + </listitem> + + <listitem> + <para><envar>PYTHONDONTWRITEBYTECODE</envar></para> + </listitem> + + <listitem> + <para><envar>PYTHONIOENCODING</envar></para> + </listitem> + + <listitem> + <para><envar>PYTHONUSERBASE</envar></para> + </listitem> + + <listitem> + <para><envar>PYTHONHASHSEED</envar></para> + </listitem> + </itemizedlist> + + (It appears to be a Python implementation detail beyond the control + of PL/Python that some of the environment variables listed on + the <command>python</command> man page are only effective in a + command-line interpreter and not an embedded Python interpreter.) + </para> + </sect1> +</chapter> |