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Programming Language Functions

Programming Language Functions on Base Types

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 the Postgres char16 type:

         /* 16-byte structure, passed by reference */
         typedef struct {
             char data[16];
         } char16;

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"
         #include "utils/palloc.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"  /* for char16, etc. */
         #include "utils/palloc.h" /* for palloc */
         int
         add_one(int arg)
         {
             return(arg + 1);
         }
         char16 *
         concat16(char16 *arg1, char16 *arg2)
         {
             char16 *new_c16 = (char16 *) palloc(sizeof(char16));
             memset((void *) new_c16, 0, sizeof(char16));
             (void) strncpy(new_c16, arg1, 16);
             return (char16 *)(strncat(new_c16, arg2, 16));
         }
         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);
         }

On OSF/1 we would type:

         CREATE FUNCTION add_one(int4) RETURNS int4
              AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';

         CREATE FUNCTION concat16(char16, char16) RETURNS char16
              AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';

         CREATE FUNCTION copytext(text) RETURNS text
              AS 'PGROOT/tutorial/obj/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).

Programming Language Functions on Composite Types

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"  /* for char16, etc. */
         #include "libpq-fe.h" /* for TUPLE */
         bool
         c_overpaid(TUPLE 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 char16, 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.

Caveats

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. 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 usually a good idea to include that file as well.

  • Compiling and loading your object code so that it can be dynamically loaded into Postgres always requires special flags. See Appendix A for a detailed explanation of how to do it for your particular operating system.


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