As it turns out, part of defining a new type is the definition of functions that describe its behavior. Consequently, while it is possible to define a new function without defining a new type, the reverse is not true. We therefore describe how to add new functions to Postgres before describing how to add new types. Postgres SQL provides three types of functions: query language functions (functions written in SQL) procedural language functions (functions written in, for example, PLTCL or PLSQL) programming language functions (functions written in a compiled programming language such as C) Every kind of function can take a base type, a composite type or some combination as arguments (parameters). In addition, every kind of function can return a base type or a composite type. It's easiest to define SQL functions, so we'll start with those. Examples in this section can also be found in funcs.sql and funcs.c. SQL functions execute an arbitrary list of SQL queries, returning the results of the last query in the list. SQL functions in general return sets. If their returntype is not specified as a setof, then an arbitrary element of the last query's result will be returned. The body of a SQL function following AS should be a list of queries separated by whitespace characters and bracketed within quotation marks. Note that quotation marks used in the queries must be escaped, by preceding them with two backslashes. Arguments to the SQL function may be referenced in the queries using a $n syntax: $1 refers to the first argument, $2 to the second, and so on. If an argument is complex, then a dot notation (e.g. "$1.emp") may be used to access attributes of the argument or to invoke functions. To illustrate a simple SQL function, consider the following, which might be used to debit a bank account: create function TP1 (int4, float8) returns int4 as 'update BANK set balance = BANK.balance - $2 where BANK.acctountno = $1 select(x = 1)' language 'sql'; A user could execute this function to debit account 17 by $100.00 as follows: select (x = TP1( 17,100.0)); The following more interesting example takes a single argument of type EMP, and retrieves multiple results: select function hobbies (EMP) returns set of HOBBIES as 'select (HOBBIES.all) from HOBBIES where $1.name = HOBBIES.person' language 'sql'; The simplest possible SQL function has no arguments and simply returns a base type, such as int4: CREATE FUNCTION one() RETURNS int4 AS 'SELECT 1 as RESULT' LANGUAGE 'sql'; SELECT one() AS answer; +-------+ |answer | +-------+ |1 | +-------+ Notice that we defined a target list for the function (with the name RESULT), but the target list of the query that invoked the function overrode the function's target list. Hence, the result is labelled answer instead of one. It's almost as easy to define SQL functions that take base types as arguments. In the example below, notice how we refer to the arguments within the function as $1 and $2: CREATE FUNCTION add_em(int4, int4) RETURNS int4 AS 'SELECT $1 + $2;' LANGUAGE 'sql'; SELECT add_em(1, 2) AS answer; +-------+ |answer | +-------+ |3 | +-------+ When specifying functions with arguments of composite types (such as EMP), we must not only specify which argument we want (as we did above with $1 and $2) but also the attributes of that argument. For example, take the function double_salary that computes what your salary would be if it were doubled: CREATE FUNCTION double_salary(EMP) RETURNS int4 AS 'SELECT $1.salary * 2 AS salary;' LANGUAGE 'sql'; SELECT name, double_salary(EMP) AS dream FROM EMP WHERE EMP.cubicle ~= '(2,1)'::point; +-----+-------+ |name | dream | +-----+-------+ |Sam | 2400 | +-----+-------+ Notice the use of the syntax $1.salary. Before launching into the subject of functions that return composite types, we must first introduce the function notation for projecting attributes. The simple way to explain this is that we can usually use the notation attribute(class) and class.attribute interchangably: -- -- this is the same as: -- SELECT EMP.name AS youngster FROM EMP WHERE EMP.age < 30 -- SELECT name(EMP) AS youngster FROM EMP WHERE age(EMP) < 30; +----------+ |youngster | +----------+ |Sam | +----------+ As we shall see, however, this is not always the case. This function notation is important when we want to use a function that returns a single instance. We do this by assembling the entire instance within the function, attribute by attribute. This is an example of a function that returns a single EMP instance: CREATE FUNCTION new_emp() RETURNS EMP AS 'SELECT \'None\'::text AS name, 1000 AS salary, 25 AS age, \'(2,2)\'::point AS cubicle' LANGUAGE 'sql'; In this case we have specified each of the attributes with a constant value, but any computation or expression could have been substituted for these constants. Defining a function like this can be tricky. Some of the more important caveats are as follows: The target list order must be exactly the same as that in which the attributes appear in the CREATE TABLE statement (or when you execute a .* query). You must typecast the expressions (using ::) very carefully or you will see the following error: WARN::function declared to return type EMP does not retrieve (EMP.*) When calling a function that returns an instance, we cannot retrieve the entire instance. We must either project an attribute out of the instance or pass the entire instance into another function. SELECT name(new_emp()) AS nobody; +-------+ |nobody | +-------+ |None | +-------+ The reason why, in general, we must use the function syntax for projecting attributes of function return values is that the parser just doesn't understand the other (dot) syntax for projection when combined with function calls. SELECT new_emp().name AS nobody; WARN:parser: syntax error at or near "." Any collection of commands in the SQL query language can be packaged together and defined as a function. The commands can include updates (i.e., INSERT, UPDATE, and DELETE) as well as SELECT queries. However, the final command must be a SELECT that returns whatever is specified as the function's returntype. CREATE FUNCTION clean_EMP () RETURNS int4 AS 'DELETE FROM EMP WHERE EMP.salary <= 0; SELECT 1 AS ignore_this' LANGUAGE 'sql'; SELECT clean_EMP(); +--+ |x | +--+ |1 | +--+ Procedural languages aren't built into Postgres. They are offered by loadable modules. Please refer to the documentation for the PL in question for details about the syntax and how the AS clause is interpreted by the PL handler. There are two procedural languages available with the standard Postgres distribution (PLTCL and PLSQL), and other languages can be defined. Refer to for more information. Internal functions are functions written in C which have been statically linked into the Postgres backend process. The AS clause gives the C-language name of the function, which need not be the same as the name being declared for SQL use. (For reasons of backwards compatibility, an empty AS string is accepted as meaning that the C-language function name is the same as the SQL name.) Normally, all internal functions present in the backend are declared as SQL functions during database initialization, but a user could use CREATE FUNCTION to create additional alias names for an internal function. Functions written in C can be compiled into dynamically loadable objects, and used to implement user-defined SQL functions. The first time the user defined function is called inside the backend, the dynamic loader loads the function's object code into memory, and links the function with the running Postgres executable. The SQL syntax for CREATE FUNCTION links the SQL function to the C source function in one of two ways. If the SQL function has the same name as the C source function the first form of the statement is used. The string argument in the AS clause is the full pathname of the file that contains the dynamically loadable compiled object. If the name of the C function is different from the desired name of the SQL function, then the second form is used. In this form the AS clause takes two string arguments, the first is the full pathname of the dynamically loadable object file, and the second is the link symbol that the dynamic loader should search for. This link symbol is just the function name in the C source code. After it is used for the first time, a dynamically loaded, user function is retained in memory, and future calls to the function only incur the small overhead of a symbol table lookup. The string which specifies the object file (the string in the AS clause) should be the full path of the object code file for the function, bracketed by quotation marks. If a link symbol is used in the AS clause, the link symbol should also be bracketed by single quotation marks, and should be exactly the same as the name of the function in the C source code. On Unix systems the command nm will print all of the link symbols in a dynamically loadable object. (Postgres will not compile a function automatically; it must be compiled before it is used in a CREATE FUNCTION command. See below for additional information.) The following table gives the C type required for parameters in the C functions that will be loaded into Postgres. The "Defined In" column gives the actual header file (in the .../src/backend/ directory) that the equivalent C type is defined. However, if you include utils/builtins.h, these files will automatically be included. Equivalent C Types Built-In Type C Type Defined In abstime AbsoluteTime utils/nabstime.h bool bool include/c.h box (BOX *) utils/geo-decls.h bytea (bytea *) include/postgres.h char char N/A cid CID include/postgres.h datetime (DateTime *) include/c.h or include/postgres.h int2 int2 include/postgres.h int2vector (int2vector *) include/postgres.h int4 int4 include/postgres.h float4 float32 or (float4 *) include/c.h or include/postgres.h float8 float64 or (float8 *) include/c.h or include/postgres.h lseg (LSEG *) include/geo-decls.h name (Name) include/postgres.h oid oid include/postgres.h oidvector (oidvector *) include/postgres.h path (PATH *) utils/geo-decls.h point (POINT *) utils/geo-decls.h regproc regproc or REGPROC include/postgres.h reltime RelativeTime utils/nabstime.h text (text *) include/postgres.h tid ItemPointer storage/itemptr.h timespan (TimeSpan *) include/c.h or include/postgres.h tinterval TimeInterval utils/nabstime.h uint2 uint16 include/c.h uint4 uint32 include/c.h xid (XID *) include/postgres.h Internally, Postgres regards a base type as a "blob of memory." The user-defined functions that you define over a type in turn define the way that Postgres can operate on it. That is, Postgres will only store and retrieve the data from disk and use your user-defined functions to input, process, and output the data. Base types can have one of three internal formats: pass by value, fixed-length pass by reference, fixed-length pass by reference, variable-length By-value types can only be 1, 2 or 4 bytes in length (even if your computer supports by-value types of other sizes). Postgres itself only passes integer types by value. You should be careful to define your types such that they will be the same size (in bytes) on all architectures. For example, the long type is dangerous because it is 4 bytes on some machines and 8 bytes on others, whereas int type is 4 bytes on most Unix machines (though not on most personal computers). A reasonable implementation of the int4 type on Unix machines might be: /* 4-byte integer, passed by value */ typedef int int4; On the other hand, fixed-length types of any size may be passed by-reference. For example, here is a sample implementation of a Postgres type: /* 16-byte structure, passed by reference */ typedef struct { double x, y; } Point; Only pointers to such types can be used when passing them in and out of Postgres functions. Finally, all variable-length types must also be passed by reference. All variable-length types must begin with a length field of exactly 4 bytes, and all data to be stored within that type must be located in the memory immediately following that length field. The length field is the total length of the structure (i.e., it includes the size of the length field itself). We can define the text type as follows: typedef struct { int4 length; char data[1]; } text; Obviously, the data field is not long enough to hold all possible strings; it's impossible to declare such a structure in C. When manipulating variable-length types, we must be careful to allocate the correct amount of memory and initialize the length field. For example, if we wanted to store 40 bytes in a text structure, we might use a code fragment like this: #include "postgres.h" ... char buffer[40]; /* our source data */ ... text *destination = (text *) palloc(VARHDRSZ + 40); destination->length = VARHDRSZ + 40; memmove(destination->data, buffer, 40); ... Now that we've gone over all of the possible structures for base types, we can show some examples of real functions. Suppose funcs.c look like: #include <string.h> #include "postgres.h" /* By Value */ int add_one(int arg) { return(arg + 1); } /* By Reference, Fixed Length */ Point * makepoint(Point *pointx, Point *pointy ) { Point *new_point = (Point *) palloc(sizeof(Point)); new_point->x = pointx->x; new_point->y = pointy->y; return new_point; } /* By Reference, Variable Length */ text * copytext(text *t) { /* * VARSIZE is the total size of the struct in bytes. */ text *new_t = (text *) palloc(VARSIZE(t)); memset(new_t, 0, VARSIZE(t)); VARSIZE(new_t) = VARSIZE(t); /* * VARDATA is a pointer to the data region of the struct. */ memcpy((void *) VARDATA(new_t), /* destination */ (void *) VARDATA(t), /* source */ VARSIZE(t)-VARHDRSZ); /* how many bytes */ return(new_t); } text * concat_text(text *arg1, text *arg2) { int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ; text *new_text = (text *) palloc(new_text_size); memset((void *) new_text, 0, new_text_size); VARSIZE(new_text) = new_text_size; strncpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ); strncat(VARDATA(new_text), VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ); return (new_text); } On OSF/1 we would type: CREATE FUNCTION add_one(int4) RETURNS int4 AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c'; CREATE FUNCTION makepoint(point, point) RETURNS point AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c'; CREATE FUNCTION concat_text(text, text) RETURNS text AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c'; CREATE FUNCTION copytext(text) RETURNS text AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c'; On other systems, we might have to make the filename end in .sl (to indicate that it's a shared library). Composite types do not have a fixed layout like C structures. Instances of a composite type may contain null fields. In addition, composite types that are part of an inheritance hierarchy may have different fields than other members of the same inheritance hierarchy. Therefore, Postgres provides a procedural interface for accessing fields of composite types from C. As Postgres processes a set of instances, each instance will be passed into your function as an opaque structure of type TUPLE. Suppose we want to write a function to answer the query * SELECT name, c_overpaid(EMP, 1500) AS overpaid FROM EMP WHERE name = 'Bill' or name = 'Sam'; In the query above, we can define c_overpaid as: #include "postgres.h" #include "executor/executor.h" /* for GetAttributeByName() */ bool c_overpaid(TupleTableSlot *t, /* the current instance of EMP */ int4 limit) { bool isnull = false; int4 salary; salary = (int4) GetAttributeByName(t, "salary", &isnull); if (isnull) return (false); return(salary > limit); } GetAttributeByName is the Postgres system function that returns attributes out of the current instance. It has three arguments: the argument of type TUPLE passed into the function, the name of the desired attribute, and a return parameter that describes whether the attribute is null. GetAttributeByName will align data properly so you can cast its return value to the desired type. For example, if you have an attribute name which is of the type name, the GetAttributeByName call would look like: char *str; ... str = (char *) GetAttributeByName(t, "name", &isnull) The following query lets Postgres know about the c_overpaid function: * CREATE FUNCTION c_overpaid(EMP, int4) RETURNS bool AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c'; While there are ways to construct new instances or modify existing instances from within a C function, these are far too complex to discuss in this manual. We now turn to the more difficult task of writing programming language functions. Be warned: this section of the manual will not make you a programmer. You must have a good understanding of C (including the use of pointers and the malloc memory manager) before trying to write C functions for use with Postgres. While it may be possible to load functions written in languages other than C into Postgres, this is often difficult (when it is possible at all) because other languages, such as FORTRAN and Pascal often do not follow the same calling convention as C. That is, other languages do not pass argument and return values between functions in the same way. For this reason, we will assume that your programming language functions are written in C. C functions with base type arguments can be written in a straightforward fashion. The C equivalents of built-in Postgres types are accessible in a C file if PGROOT/src/backend/utils/builtins.h is included as a header file. This can be achieved by having #include <utils/builtins.h> at the top of the C source file. The basic rules for building C functions are as follows: Most of the header (include) files for Postgres should already be installed in PGROOT/include (see Figure 2). You should always include -I$PGROOT/include on your cc command lines. Sometimes, you may find that you require header files that are in the server source itself (i.e., you need a file we neglected to install in include). In those cases you may need to add one or more of -I$PGROOT/src/backend -I$PGROOT/src/backend/include -I$PGROOT/src/backend/port/<PORTNAME> -I$PGROOT/src/backend/obj (where <PORTNAME> is the name of the port, e.g., alpha or sparc). When allocating memory, use the Postgres routines palloc and pfree instead of the corresponding C library routines malloc and free. The memory allocated by palloc will be freed automatically at the end of each transaction, preventing memory leaks. Always zero the bytes of your structures using memset or bzero. Several routines (such as the hash access method, hash join and the sort algorithm) compute functions of the raw bits contained in your structure. Even if you initialize all fields of your structure, there may be several bytes of alignment padding (holes in the structure) that may contain garbage values. Most of the internal Postgres types are declared in postgres.h, so it's a good idea to always include that file as well. Including postgres.h will also include elog.h and palloc.h for you. Compiling and loading your object code so that it can be dynamically loaded into Postgres always requires special flags. See for a detailed explanation of how to do it for your particular operating system. More than one function may be defined with the same name, as long as the arguments they take are different. In other words, function names can be overloaded. A function may also have the same name as an attribute. In the case that there is an ambiguity between a function on a complex type and an attribute of the complex type, the attribute will always be used. As of Postgres v6.6, the alternative form of the AS clause for the SQL CREATE FUNCTION command decouples the SQL function name from the function name in the C source code. This is now the preferred technique to accomplish function overloading. For functions written in C, the SQL name declared in CREATE FUNCTION must be exactly the same as the actual name of the function in the C code (hence it must be a legal C function name). There is a subtle implication of this restriction: while the dynamic loading routines in most operating systems are more than happy to allow you to load any number of shared libraries that contain conflicting (identically-named) function names, they may in fact botch the load in interesting ways. For example, if you define a dynamically-loaded function that happens to have the same name as a function built into Postgres, the DEC OSF/1 dynamic loader causes Postgres to call the function within itself rather than allowing Postgres to call your function. Hence, if you want your function to be used on different architectures, we recommend that you do not overload C function names. There is a clever trick to get around the problem just described. Since there is no problem overloading SQL functions, you can define a set of C functions with different names and then define a set of identically-named SQL function wrappers that take the appropriate argument types and call the matching C function. Another solution is not to use dynamic loading, but to link your functions into the backend statically and declare them as INTERNAL functions. Then, the functions must all have distinct C names but they can be declared with the same SQL names (as long as their argument types differ, of course). This way avoids the overhead of an SQL wrapper function, at the cost of more effort to prepare a custom backend executable. (This option is only available in version 6.5 and later, since prior versions required internal functions to have the same name in SQL as in the C code.)