Re: PL/PgSQL discussion

jwieck@debis.com (Jan Wieck) wrote:

>     But I would like to have some discussion on language  itself.
>     So  I wrote down what I had in mind. The document is appended
>     below.
>
>     Please comment/suggest !
>
>     Someone gave a hint about global variables existing during  a
>     session.   What  is  a  session  than?  One  transaction? The
>     backends lifetime?

I think it is a time from connect to disconnect, which currently equals to backends
lifetime, but may in future be shorter, if we will implement a backend pool for
quick-starting servers.

> And should global variables be visible by
>     more  than one function?

How are they global then ?

I think that global variables are something like DateStyle is now.

> I vote for NO! In that case we need
>     something like packages of functions that share globals.

Or we need local temporary tables.

>                                 PL/pgSQL
>              A procedural language for the PostgreSQL RDBMS
>                                 1st draft
>
>                       Jan Wieck <jwieck@debis.com>
>
>     Notice
>
>         This  document  is  for  the postgresql-hackers community for
>         completing  the  syntax  specification   of   PL/pgSQL.   The
>         extension module described here doesn't exist up to now!
>
>     Preface
>
>         PL/pgSQL  is  a procedural language based on SQL designed for
>         the PostgreSQL database system.
>
>         The extensibility features of PostgreSQL are mostly based  on
>         the  ability  to  define  functions  for  various operations.
>         Functions could have been written in PostgreSQL's SQL dialect
>         or  in the C programming language. Functions written in C are
>         compiled into a shared object  and  loaded  by  the  database
>         backend  process  on  demand.   Also  the trigger features of
>         PostgreSQL are based on functions but required the use of the
>         C language.
>
>         Since  version  6.3  PostgreSQL  supports  the  definition of
>         procedural languages. In the case of a  function  or  trigger
>         procedure  defined in a procedural language, the database has
>         no builtin knowlege how to  interpret  the  functions  source
>         text. Instead, the function and trigger calls are passed into
>         a handler that  knows  the  details  of  the  language.   The
>         handler  itself is function compiled into a shared object and
>         loaded on demand.
>
>     Overview
>
>         The PL/pgSQL language is case insensitive. All  keywords  and
>         identifiers can be used in upper-/lowercase mixed.
>
>         PL/pgSQL is a block oriented language. A block is defined as
>
>             [<<label>>]
>             [DECLARE
>                 -- declarations]
>             BEGIN
>                 -- statements
>             END;



>
>
>         There  can  be  any  number  of  subblocks  in the statements
>         section of a block. Subblocks can be used to  hide  variables
>         from  outside a block of statements (see Scope and visability
>         below).

I think that subblock should also be used as the extent of IF, FOR ... LOOP, WHILE and
other such statements.

Then we would not need the END IF, END LOOP etc.

>
>
>     Comments
>
>         There are two types of comments in PL/pgSQL.  A  double  dash
>         '--'  starts a comment that extends to the end of the line. A
>         '/*' starts a block comment that extends to  the  next  '*/'.
>         Block comments cannot be nested, but double dash comments can
>         be enclosed into a block comment.

And vice versa : block comment delimiters can be commented out by --

>
>
>     Declarations
>
>         All variables, rows and records  used  in  a  block  or  it's
>         subblocks must be declared in the declarations section of the
>         block.   The   parameters   given   to   the   function   are
>         automatically  declared  with  the usual identifiers $n.

How hard would it bet to have named parameters, or why must we use alias?

>         The declarations have the following syntax:
>
>             <name> [CONSTANT] <type> [NOT NULL]
>                                      [DEFAULT | := <value>];
>
>                 Declares a variable of the  specified  type.  If  the
>                 variable is declared as CONSTANT, the value cannot be
>                 changed. If NOT NULL is specified, an assignment of a
>                 NULL  value  results  in  a  runtime error. Since the
>                 default value of a variable is the  SQL  NULL  value,
>                 all  variables  declared as NOT NULL must also have a
>                 default value.
>
>                 The default value is evaluated at the actual function
>                 call. So assigning 'now' to an abstime varable causes
>                 the variable to have the time of the actual  function
>                 call, not when the function was compiled.
>
>             <name> <class>%ROWTYPE;
>
>                 Declares a row with the structure of the given class.
>                 Class must be an existing table- or viewname  of  the
>                 database.  The  fields of the row are accessed in the
>                 dot notation. Parameters  to  a  procedure  could  be
>                 tuple   types.   In   that   case  the  corresponding
>                 identifier $n  will  be  a  rowtype.  Only  the  user
>                 attributes  and  the oid of a tuple are accessible in
>                 the row. There must be  no  whitespaces  between  the
>                 classname, the percent and the ROWTYPE keyword.
>
>             <name> RECORD;
>
>                 Records  are  similar  to  rowtypes, but they have no
>                 predefined structure and it's impossible to assign  a
>                 value  into them. They are used in selections and FOR
>                 loops to hold one actual database tuple from a select
>                 operation.  One  and  the  same record can be used in
>                 different selections (but not in nested ones).
>
>             <name> ALIAS FOR $n;
>
>                 For better readability of the code it's  possible  to
>                 define  an  alias  for  a positional parameter to the
>                 function.
>
>     Datatypes
>
>         The type of a variable can be any of the existing data  types
>         of the database. <type> above is defined as:
>
>                 postgesql-basetype
>             or  variable%TYPE
>             or  rowtype.field%TYPE
>             or  class.field%TYPE
>
>         As  for the rowtype declaration, there must be no whitespaces
>         between the classname, the percent and the TYPE keyword.
>
>     Expressions
>
>         All expressions used in  PL/pgSQL  statements  are  processed
>         using  the  backends  executor. Since even a constant looking
>         expression  can  have  a  totally  different  meaning  for  a
>         particular data type (as 'now' for abstime), it is impossible
>         for the PL/pgSQL parser  to  identify  real  constant  values
>         other than the NULL keyword. The expressions are evaluated by
>         internally executing a query
>
>             SELECT <expr>
>
>         over the  SPI  manager.  In  the  expression,  occurences  of
>         variable  identifiers  are  substituted by parameters and the
>         actual values from the variables are passed to  the  executor
>         as  query  parameters. All the expressions used in a PL/pgSQL
>         function are only prepared and saved once.
>
>     Statements
>
>         Anything not understood by the parser as specified below will
>         be  put  into a query and sent down to the database engine to
>         execute.  The resulting query should not return any data.
>
>         Assignment
>
>             An assignment of a value to a variable or  rowtype  field
>             is written as:
>
>                 <identifier> := <expr>;
>
>             If  the  expressions  result  data type doesn't match the
>             variables data type, or the variables atttypmod value  is
>             known  (as  for  char(20)),  the  result  value  will  be
>             implicitly casted by  the  PL/pgSQL  executor  using  the
>             result  types  output-  and  the  variables  type  input-
>             functions.  Note that this could  potentially  result  in
>             runtime errors generated by the types input functions.
>
>             An  assignment  of  a complete selection into a record or
>             rowtype can be done as:
>
>                 SELECT targetlist INTO <recname|rowname> FROM fromlist;
>
>             If a rowtype is used as target, the  selected  targetlist
>             must  exactly  match  the  structure  of the rowtype or a
>             runtime error occurs.  The fromlist can  be  followed  by
>             any  valid qualification, grouping, sorting etc. There is
>             a  special  condition  [NOT]  FOUND  that  can  be   used
>             immediately  after a SELECT INTO to check if the data has
>             been found.
>
>                 SELECT * INTO myrec FROM EMP WHERE empname = myname;
>                 IF NOT FOUND THEN
>                     ELOG ERROR 'employee %s not found' myname;
>                 END IF;
>
>             In addition, the select statement must  not  return  more
>             that  one  row.  If multiple rows are returned, a runtime
>             error will be generated.
>
>         Returning from the function
>
>                 RETURN <expr>;
>
>             The function terminates and the value of <expr>  will  be
>             returned  to  the  upper executor.  The return value of a
>             function cannot be undefined.  If control reaches the end
>             of  the  toplevel block of the function without hitting a
>             RETURN statement, a runtime error will occur.

What can <expr> be?

Possibilities: null, single value, record, recordset

AFAIK, recordsets are usually returned by more strange constructs, like haveing some
kinds of breakpoints inside the loop that either returns a record or some EOF token.

>                     A  conditional  loop  that  is  executed  as  long as the
>             evaluation of <expr> returns true.
>
>                 [<<label>>]
>                 FOR <name> IN [REVERSE] <expr>..<expr> LOOP
>                     -- statements

>                 END LOOP.

Perhaps PL/PgSQL should have some notation for specifying immediate lists of other kinds
of values as well,perhaps like ['aa','bb','cc'], so that one cold loop over not only
integers. Perhaps even with type specifiers, like [::time '22.01', '13.47.05', '15.20']

>
>
>             A loop that iterates over a range of integer values.  The
>             variable  <name> is automatically created as type integer
>             and exists only inside  the  loop.  The  two  expressions
>             giving  the  lower  and  upper  bound  of  the  range are
>             evaluated only when entering the loop. The iteration step
>             is 1.
>
>                 FOR <recname|rowname> IN <select_clause> LOOP
>                     -- statements
>                 END LOOP;
>
>             The record or row is assigned all the rows resulting from
>             the select clause and the statements executed  for  each.
>             If  the  loop  is  terminated with an EXIT statement, the
>             last accessed row is still accessible in  the  record  or
>             rowtype.
>
>                 EXIT [label] [WHEN <expr>];
>
>             If  no  label given, the innermost loop is terminated and
>             the statement following END LOOP  is  executed  next.  If
>             label is given, it must be the label of the current or an
>             upper level of nested loops or blocks.   Then  the  named
>             loop  or  block  is terminated and control continues with
>             the statement after the loops/blocks corresponding END.
>

Keep up the nice work!

Hannu