path: root/src/interfaces/ecpg/doc/ecpg.texinfo

\input texinfo   @c -*-texinfo-*-
@c %**start of header
@setfilename ecpg.info
@settitle Ecpg - Embedded SQL in C for PostgreSQL
@setchapternewpage odd
@c %**end of header

@ifinfo
This file documents an embedded SQL in C package for PostgreSQL.

Copyright 1996 Linus Tolke

Permission is granted to copy and use in the same way as you are allowed
to copy and use the rest of the PostgreSQL.
@end ifinfo

@c  This title page illustrates only one of the
@c  two methods of forming a title page.

@titlepage
@title ECPG
@subtitle Embedded SQL in C for PostgreSQL
@author Linus Tolke

@c  The following two commands
@c  start the copyright page.
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1996-1997 Linus Tolke, 1998 Michael Meskes

Published by Linus Tolke

Permission is granted to copy and use in the same way as you are allowed
to copy and use the rest of the PostgreSQL.
@end titlepage

@node Top, Why embedded SQL, (dir), (dir)
@comment  node-name,  next,  previous,  up

@ifinfo
Ecpg is an embedded sql preprocessor for C and library for PostgresSQL

It is written by Linus Tolke <linus@@epact.se> and Michael Meskes <meskes@@debian.org>.

@end ifinfo

@menu
* Why embedded SQL::            
* Simple description of the concept::  
* How to use it::               
* Limitations::                 
* Porting from other DBMSs::    
* Installation::                
* Index::                       
* For the developer::           

 --- The Detailed Node Listing ---

How to use it

* Preprocessor::                
* Library::                     
* Error handling::              

For the developer

* To do list::                  
* The preprocessor::            
* A complete example::          
* The library::                 
@end menu

@node Why embedded SQL, Simple description of the concept, Top, Top
@comment  node-name,  next,  previous,  up
@chapter Why embedded SQL

Embedded SQL has some small advantages over other ways to handle SQL
queries. It takes care of all the tidious moving of information to and
from variables in your c-program.

There is an ANSI-standard describing how the embedded language should
work. Most embedded sql preprocessors I have seen and heard of makes
extensions so it is difficult to obtain portability even between them
anyway. I have not read the standard but I hope that my implementation
does not deviate to much and that it would be possible to port programs
with embedded sql written for other DBMS:s to PostgreSQL and thus
promoting the spirit of free software.


@node Simple description of the concept, How to use it, Why embedded SQL, Top
@comment  node-name,  next,  previous,  up
@chapter Simple description of the concept

You write your program in C with some special sql things.
For declaring variables that can be used in SQL statements you need to
put them in a special declare section.
You use a special syntax for the sql queries.

Before compiling you run the file through the embedded sql c
preprocessor and it converts the SQL statements you used to function
calls with the variables used as arguments. Both variables that are used
as input to the SQL statements and variables that will contain the
result are passed.

Then you compile and at link time you link with a special library that
contains the functions used. These functions (actually it is mostly one
single function) fetches the information from the arguments, performs
the SQL query using the ordinary interface (pq) and puts back
the result in the arguments dedicated for output.

Then you run your program and when the control arrives to the SQL
statement the SQL statement is performed against the database and you
can continue with the result.


@node How to use it, Limitations, Simple description of the concept, Top
@comment  node-name,  next,  previous,  up
@chapter How to use it

This chapter describes how to use the ECPG tool.

@menu
* Preprocessor::                
* Library::                     
* Error handling::              
@end menu

@node Preprocessor, Library, How to use it, How to use it
@comment  node-name,  next,  previous,  up
@section Preprocessor

@cindex preprocessor
@cindex @code{ecpg}
The preprocessor is called @code{ecpg}. After installation it resides in
the postgres @code{bin} directory. 

@node Library, Error handling, Preprocessor, How to use it
@section Library

@cindex library functions
@cindex @code{libecpg.a}
@cindex @code{-lecpg}
The library is called @code{libecpg.a} resp.  @code{libecpg.so}. The library
used the pq library for the communication to the postgres server so you will
have to link your program with @code{-lecpg -lpq}.

The library has some methods that are "hidden" but that could prove very
useful sometime.

@table @asis
@item @code{ECPGdebug(int, FILE *stream)}
@cindex @code{ECPGdebug(int, FILE *stream)}
@cindex debuglogging
If this is called, with the first argument non-zero, then debuglogging is turned
on. Debuglogging is done on @code{stream}. Most SQL statement logs its
arguments and result.

The most important one (@code{ECPGdo}) that is called on all SQL
statements except @code{EXEC SQL COMMIT}, @code{EXEC SQL ROLLBACK}, 
@code{EXEC SQL CONNECT} logs both its expanded string, i.e. the string
with all the input variables inserted, and the result from the
PostgreSQL server. This can be very useful when searching for errors
in your SQL statements.

@item @code{ECPGstatus()}
@cindex @code{ECPGstatus()}
This method returns TRUE if we are connected to a database and FALSE if
not.
@end table

@node Error handling,  , Library, How to use it
@comment  node-name,  next,  previous,  up
@section Error handling

@cindex @code{sqlca.h}
@cindex @code{struct sqlca}
@cindex @code{sqlcode}
@cindex @code{error messages}
To be able to detect errors from the postgres server you include a line
like:
@example
exec sql include sqlca;
@end example
in the include section of your file. This will define a struct and a
variable with the name @code{sqlca} as following:
@example
struct sqlca @{
    int sqlcode;
    struct @{
        int sqlerrml;
        char sqlerrmc[1000];
    @} sqlerrm;
@} sqlca;
@end example

If an error occured in the last SQL statement then @code{sqlca.sqlcode}
will be non-zero. If @code{sqlca.sqlcode} is less that 0 then this is
some kind of serious error, like the database definition does not match
the query given. If it is bigger than 0 then this is a normal error like
the table did not contain the requested row.

sqlca.sqlerrm.sqlerrmc will contain a string that describes the error.
The string ends with @code{line 23.} where the line is the line number
in the source file (actually the file generated by the preprocessor but
I hope I can fix this to be the line number in the input file.)

List of errors that can occur:
@cindex error list
@table @asis
@item -1, Unsupported type %s on line %d.
Does not normally occur. This is a sign that the preprocessor has
generated something that the library does not know about. Perhaps you
are running incompatible versions of the preprocessor and the library.

@item -1, Too many arguments line %d.
@itemx -1, Too few arguments line %d.
The preprocessor has goofed up and generated some incorrect code.

@item -1, Error starting transaction line %d.
PostgreSQL signalled to us that we cannot open the connection.

@item -1, Postgres error: %s line %d.
Some PostgreSQL error. The message contains the error message from the
PostgreSQL backend.

@item 1, Data not found line %d.
This is a "normal" error that tells you that what you are quering cannot
be found or we have gone through the cursor.

@item -1, To many matches line %d.
This means that the query has returned several lines. The @code{SELECT}
you made probably was not unique.

@item -1, Not correctly formatted int type: %s line %d.
This means that the host variable is of an @code{int} type and the field
in the PostgreSQL database is of another type and contains a value that
cannot be interpreted as an @code{int}. The library uses @code{strtol}
for this conversion.

@item -1, Not correctly formatted unsigned type: %s line %d.
This means that the host variable is of an @code{unsigned int} type and
the field in the PostgreSQL database is of another type and contains a
value that cannot be interpreted as an @code{unsigned int}. The library
uses @code{strtoul} for this conversion.

@item -1, Not correctly formatted floating point type: %s line %d.
This means that the host variable is of an @code{float} type and
the field in the PostgreSQL database is of another type and contains a
value that cannot be interpreted as an @code{float}. The library
uses @code{strtod} for this conversion.

@item -1, Too few arguments line %d.
This means that PostgreSQL has returned more records than we have
matching variables. Perhaps you have forgotten a couple of the host
variables in the @code{INTO :var1,:var2}-list.

@item -1, Too many arguments line %d.
This means that PostgreSQL has returned fewer records than we have
host variables. Perhaps you have to many host variables in the 
@code{INTO :var1,:var2}-list.

@item -1, Empty query line %d.
PostgreSQL returned PGRES_EMPTY_QUERY.

@item -1, Error: %s line %d.
This means that PostgreSQL returned on of the errors
PGRES_NONFATAL_ERROR, PGRES_FATAL_ERROR or PGRES_BAD_RESPONSE. Which one
and why is explained in the message.

@item -1, Postgres error line %d.
PostgreSQL returns something that the library does not know how to
handle. This is probably because the version of PostgreSQL does not
match the version of the ecpg library.

@item -1, Error committing line %d.
Error during @code{COMMIT}. @code{EXEC SQL COMMIT} is translated to an
@code{end} operation in PostgreSQL and that is the operation that could
not be performed.

@item -1, Error rolling back line %d.
Error during @code{ROLLBACK}. @code{EXEC SQL ROLLBACK} is translated to
an @code{abort} operation in PostgreSQL and that is the operation that
could not be performed.

@item -1, ECPGconnect: could not open database %s.
The connect to the database did not work.

@end table

@node Limitations, Porting from other DBMSs, How to use it, Top
@chapter Limitations
@comment  node-name,  next,  previous,  up

What will never be included and why or what cannot be done with this
concept.

@table @asis

@item oracles single tasking possibility
@cindex single tasking
Oracle version 7.0 on AIX 3 uses the OS-supported locks on the shared
memory segments and allows the application designer to link an
application in a so called single tasking way. Instead of starting one
client process per application process both the database part and the
application part is run in the same process. In later versions of oracle
this is no longer supported.

This would require a total redesign of the postgres access model and
that effort can not justify the performance gained.

@end table


@node Porting from other DBMSs, Installation, Limitations, Top
@chapter Porting from other DBMSs
@comment  node-name,  next,  previous,  up

To be written by persons that knows the different DBMSs and that
actually does port something...

@node Installation, Index, Porting from other DBMSs, Top
@comment  node-name,  next,  previous,  up
@chapter Installation
@cindex installation

Since version 0.5 ecpg is distributed together with PostgreSQL. So you
should get your precompiler, libraries and header files compiled and
installed on the fly.

@node Index, For the developer, Installation, Top
@unnumbered Index

@printindex cp

@node For the developer,  , Index, Top
@comment  node-name,  next,  previous,  up
@chapter For the developer

This chapter is for those that wants to develop the ecpg interface. It
describes how the things work. The ambition is to make this chapter
contain things for those that want to have a look inside and the chapter
on How to use it should be enough for all normal questions.

So, read this before looking at the internals of the @code{ecpg}. If
you are not interested in how it really works, skip this chapter.

@menu
* To do list::                  
* The preprocessor::            
* A complete example::          
* The library::                 
@end menu


@node To do list, The preprocessor, For the developer, For the developer
@comment  node-name,  next,  previous,  up
@section To do list

This version the preprocessor has some flaws:
@table @asis

@item Preprocessor output
The variables should be static.

@item Preprocessor cannot do syntax checking on your SQL statements
Whatever you write is copied more or less exactly to the PostgreSQL and
you will not be able to locate your errors until run-time.

@item no restriction to strings only
The PQ interface, and most of all the PQexec function, that is used by
the ecpg relies on that the request is built up as a string. In some
cases, like when the data contains the null character, this will be a
serious problem.

@item error codes
There should be different error numbers for the different errors instead
of just -1 for them all.

@item library functions
to_date et al.

@item records
@cindex records
Possibility to define records or @code{struct}s in the declare section
in a way that the record can be filled from one row in the database.

This is a simpler way to handle an entire row at a time.

@item array operations
@cindex array operations
Oracle has array operations that enhances speed. When implementing it in
@code{ecpg} it is done for compatibility reasons only. For them to
improve speed would require a lot more insight in the postgres internal
mechanisms than I possess.

@item indicator variables
@cindex indicator variables
@cindex @code{VARCHAR2}
Oracle has indicator variables that tell if a value is @code{null} or if
it is empty. This largely simplifies array operations and provides for a
way to hack around some design flaws in the handling of @code{VARCHAR2}
@footnote{like that an empty string isn't distinguishable from a
@code{null} value}. I am not sure if this is an Oracle extension or part
of the ANSI standard.

@item typedefs
@cindex typedef
As well as complex types like records and arrays, typedefs would be
a good thing to take care of.

@item conversion of scripts
@cindex conversion of scripts
To set up a database you need a few scripts with table definitions and
other configuration parameters. If you have these scripts for an old
database you would like to just apply them to get a postgres database
that works in the same way.

The functionality could be accomplished with some conversion scripts.
Speed will never be accomplished in this way. To do this you need a
bigger insight in the database construction and the use of the database
than could be realised in a script.

@end table



@node The preprocessor, A complete example, To do list, For the developer
@comment  node-name,  next,  previous,  up
@section The preprocessor

First four lines are written to the output. Two comments and two include
lines necessary for the interface to the library. 

Then the preprocessor works in one pass only reading the input file and
writing to the output as it goes along. Normally it just echoes
everything to the output without looking at it further.

When it comes to an @code{EXEC SQL} statements it interviens and
changes them depending on what iit is. The @code{EXEC SQL} statement can
be one of these:

@itemize @bullet

@item Declare sections
@cindex Declare section
Declare sections begins with
@example
exec sql begin declare section;
@end example
and ends with
@example
exec sql end declare section;
@end example
In the section only variable declarations are allowed. Every variable
declare within this section is also entered in a list of variables
indexed on their name together with the corresponding type.

The declaration is echoed to the file to make the variable a normal
C-variable also.

The special types VARCHAR and VARCHAR2 are converted into a named struct
for every variable. A declaration like:
@example
VARCHAR var[180];
@end example
is converted into
@example
struct varchar_var @{ int len; char arr[180]; @} var;
@end example


@item Include statements
@cindex Include statement
An include statement looks like:
@example
exec sql include filename;
@end example
It is converted into
@example
#include <filename.h>
@end example

@item Connect statement
@cindex Connect statement
A connect statements looks like:
@example
exec sql connect 'databasename';
@end example
That statement is converted into
@example
ECPGconnect("databasename");
@end example

@item Open cursor statement
@cindex Open cursor statement
An open cursor statement looks like:
@example
exec sql open blablabla;
@end example
and is ignore and not copied from the output.

@item Commit statement
@cindex Commit statement
A commit statement looks like
@example
exec sql commit;
@end example
and is translated on the output to
@example
ECPGcommit(__LINE__);
@end example

@item Rollback statement
@cindex Rollback statement
A rollback statement looks like
@example
exec sql rollback;
@end example
and is translated on the output to
@example
ECPGrollback(__LINE__);
@end example

@item Other statements
Other SQL statements are other statements that start with 
@code{exec sql} and ends with @code{;}. Everything inbetween is treated
as an sql statement and parsed for variable substitution.

Variable substitution occur when a symbol starts with a colon
(@code{:}). Then a variable with that name is found among the variables
that were previously declared within a declare section and depending on
whether or not the SQL statements knows it to be a variable for input or
output the pointers to the variables are written to the output to allow
for access by the function.

For every variable that is part of the SQL request the function gets
another five arguments.
@enumerate
@item The type as a special symbol
@item A pointer to the value
@item The size of the variable if it is a varchar
@item Number of elements in the array (for array fetches)
@item The offset to the next element in the array (for array fetches)
@end enumerate
Since the array fetches are not implemented yet the two last arguments
are not really important. They could perhaps have been left out.

@end itemize


@node A complete example, The library, The preprocessor, For the developer
@comment  node-name,  next,  previous,  up
@section A complete example
Here is a complete example describing the output of the preprocessor:
@example
exec sql begin declare section;
int index;
int result;
exec sql end declare section;
...
    exec sql select res into :result from mytable where index = :index;
@end example
is translated into:
@example
/* These two include files are added by the preprocessor */
#include <ecpgtype.h>
#include <ecpglib.h>
/* exec sql begin declare section */

 int index;
 int result;
/* exec sql end declare section */

...
    ECPGdo(__LINE__, "select res from mytable where index = ;;", 
           ECPGt_int,&index,0,0,sizeof(int), 
           ECPGt_EOIT, 
           ECPGt_int,&result,0,0,sizeof(int), 
           ECPGt_EORT );
@end example
(the indentation in this manual is added for readability and not
something that the preprocessor can do.)


@node The library,  , A complete example, For the developer
@comment  node-name,  next,  previous,  up
@section The library
The most important function in the library is the @code{ECPGdo}
function. It takes a variable amount of arguments. Hopefully we wont run
into machines with limits on the amount of variables that can be
accepted by a varchar function. This could easily add up to 50 or so
arguments.

The arguments are:
@table @asis
@item A line number
This is a line number for the original line used in error messages only.
@item A string
This is the sql request that is to be issued. This request is modified
by the input variables, i.e. the variables that where not known at
compile time but are to be entered in the request. Where the variables
should go the string contains @code{;;}.
@item Input variables
As described in the section about the preprocessor every input variable
gets five arguments.
@item ECPGt_EOIT
An enum telling that there are no more input variables.
@item Output variables
As described in the section about the preprocessor every input variable
gets five arguments. These variables are filled by the function.
@item ECPGt_EORT
An enum telling that there are no more variables.
@end table

All the SQL statements are performed in one transaction unless you issue
a commit transaction. This works so that the first transaction or the
first after a commit or rollback always begins a transaction.

To be completed: entries describing the other entries.


@contents
@bye