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Pragana's Tcl Guide


Old notes


Integrating with a database

PostgreSQL is a nice client/server database tool that's gaining momentum those days.
Why? For it's open source status, SQL standards compliance, easy interfacing with languages and other tools. Tcl is already integrated into this technology in several ways. The most useful are pgtcl and pl/tcl.
The first interface (pgtcl) allows users submit queries to the engine and do everything inside a tcl/tk script. The second (pl/tcl) does the opposite: inside postgresql queries, we can use tcl procedures, instead of lower-level 'C', as a means to include procedural power in our queries.

We are going to look this week at a simple use for the pgtcl interface. Let us leave pl/tcl to another time.

Programming with pgtcl is very easy. We begin opening a connection to our postgres server at the default port 5432., (proc changeDB) and saving the return code (something like a file handle in open). Of course, if another connection was in use, we close it before opening the new connection. (We could have several connections at a time, anyway)

All queries go through pg_exec with the connection and the text of the query as arguments. This function returns a result that can be analysed further with pg_result in several ways. In our example, we process the query result only with -numTuples to get how many tuples we got back from the server, and -getTuple to read one tuple  at a time.
There are functions to retrieve everything as a tcl array,  to execute a tcl procedure with each tuple returned, and more. Please check the documents in the PostgreSQL release to find out.

To get all tables and fields available is a matter of query the system catalogs. This is done in the procedure showTables.
If you want to go further, please look for my program sqlforms at Sunsite. It allows you visually design a form with entries and labels (also lines, rectangles and other items) and assign table attributes to each entry.

#!/usr/local/pgsql/bin/pgtksh -f

set conn ""

pack [frame .ft] [frame .fm] [frame .fb] -side top
pack [label .ft.l -font {Helvetica 14 bold} -text database] \
    [entry .ft.e -font {Helvetica 14 bold} -width 40 -textvariable newdb] \
    [button .ft.tabelas -font {Helvetica 14 bold} -text tabelas -command showTa
    [button .ft.clear -font {Helvetica 14 bold} -text limpa -command {
        .fb.t delete 0.0 end}] \
    [button .ft.exit -font {Helvetica 14 bold} -text exit -command {
        if {$conn != ""} { pg_disconnect $conn } ;destroy . }] -side left
pack [label .fm.l -font {Helvetica 14 bold} -text query] \
    [entry .fm.e -font {Helvetica 14 bold} -width 60 -textvariable qry] -side l
pack [text .fb.t -width 90 -font {Courier 14 bold} -yscrollcommand {.fb.sb set}
    -fill both -side left -expand 1
pack [scrollbar .fb.sb -command {.fb.t yview}] -expand 1 -fill y
    
bind .ft.e <Return> changeDB
bind .fm.e <Return> {query $qry}

proc changeDB {} {
    global conn newdb
    if {$conn != ""} { pg_disconnect $conn }
    set conn [pg_connect $newdb]
    focus .fm.e
}
proc showTables {} {
    global conn
    set qry "select c.relname,a.attname,t.typname,a.attlen,c.oid,a.oid \
        from pg_class c, pg_attribute a, pg_type t \
        where c.relkind = 'r' \
            and c.relname !~ '^pg_' \
            and a.attnum > 0 \
            and a.attrelid = c.oid \
            and a.atttypid = t.oid \
        order by c.oid,a.oid"
    set res [pg_exec $conn $qry]
    set ntups [pg_result $res -numTuples]
    .fb.t insert end "\n     relname            attname             "
    .fb.t insert end "typname       attlen r(oid) a(oid)\n"
    .fb.t insert end "----------------------------------------------"
    .fb.t insert end "----------------------------------"
    for {set i 0} {$i < $ntups} {incr i} {

        set j 0
        foreach fld [pg_result $res -getTuple $i] {
            if {$j <= 2} {
                .fb.t insert end [format "%-20.20s" $fld]
            } {
                .fb.t insert end [format "%6.6s" $fld]
            }
            incr j
        }
        .fb.t insert end \n
    }
    .fb.t insert end "----------------------------------------------\n"
    .fb.t see end

}

proc query {qry} {
    global conn
    set res [pg_exec $conn $qry]
    set ntups [pg_result $res -numTuples]
    for {set i 0} {$i < $ntups} {incr i} {
        .fb.t insert end "Tupla $i:"
        .fb.t insert end [pg_result $res -getTuple $i]
        .fb.t insert end \n
    }
    .fb.t insert end "----------------------------------------------\n"
    .fb.t see end
}

To use this program, start it, fill in a database name and press <Return>. Then press the "tabelas" (tables in portuguese) button to see a listing of all tables and fields in the text widget.  You may also enter  a query
(for instance: select * from table) in the query entry and press <Return> there.  Don't put any ';' at the end.


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