package PostgreSQL::PLPerl::Call;

=head1 NAME

PostgreSQL::PLPerl::Call - Simple interface for calling SQL functions from PostgreSQL PL/Perl

=head1 SYNOPSIS

    use PostgreSQL::PLPerl::Call;

Returning single-row single-column values:

    $pi = call('pi'); # 3.14159265358979

    $net = call('network(inet)', '192.168.1.5/24'); # '192.168.1.0/24';

    $seqn = call('nextval(regclass)', $sequence_name);

    $dims = call('array_dims(text[])', '{a,b,c}');   # '[1:3]'

    # array arguments can be perl array references:
    $ary = call('array_cat(int[], int[])', [1,2,3], [2,1]); # '{1,2,3,2,1}'

Returning multi-row single-column values:

    @ary = call('generate_series(int,int)', 10, 15); # (10,11,12,13,14,15)

Returning single-row multi-column values:

    # assuming create function func(int) returns table (r1 text, r2 int) ...
    $row = call('func(int)', 42); # returns hash ref { r1=>..., r2=>... }

Returning multi-row multi-column values:

    @rows = call('pg_get_keywords'); # ({...}, {...}, ...)

Alternative method-call syntax:

    $pi   = SP->pi();
    $seqn = SP->nextval($sequence_name);

=head1 DESCRIPTION

The C<call> function provides a simple efficient way to call SQL functions
from PostgreSQL PL/Perl code.

The first parameter is a I<signature> that specifies the name of the function
to call and, optionally, the types of the arguments.

Any further parameters are used as argument values for the function being called.

=head2 Signature

The first parameter is a I<signature> that specifies the name of the function.
The name should be given in the same way it would in an SQL statement, so
if identifier quoting is needed it should be specified in the already quoted form.

Immediately after the function name, in parenthesis, a comma separated list of
type names can be given. For example:

    'pi()'
    'generate_series(int,int)'
    'array_cat(int[], int[])'
    'myschema.myfunc(date, float8)'

The types specify how the I<arguments> to the call should be interpreted.
They don't have to exactly match the types used to declare the function you're
calling.

You also don't have to specify types for I<all> the arguments, just the
left-most arguments that need types.

=head2 Array Arguments

The argument value corresponding to a type that contains 'C<[]>' can be a
string formated as an array literal, or a reference to a perl array. In the
later case the array reference is automatically converted into an array literal
using the C<encode_array_literal()> function.

=head2 Varadic Functions

Functions with C<varadic> arguments can be called with a fixed number of
arguments by repeating the type name in the signature the same number of times.
For example, given:

    create function vary(VARADIC int[]) as ...

you can call that function with three arguments using:

    call('vary(int,int,int)', $int1, $int2, $int3);

Alternatively, you can append the string 'C<...>' to the last type in the
signature to indicate that the argument is varadic. For example:

    call('vary(int...)', @ints);

==head2 Method-call Syntax

An alternative syntax can be used for making calls:

    SP->function_name(@args)

For example:

    $pi   = SP->pi();
    $seqn = SP->nextval($sequence_name);

Using this form you can't easily specify a schema name or argument types, and
you can't call varadic functions.

If cases where a signature is needed you might get a somewhat confusing error
message. For example:

    SP->generate_series(10,20);

fails with the error "there is no parameter $1". The underlying problem is that
C<generate_series> is a polymorphic function: different versions of the
function are executed depending on the type of the arguments.

==head2 Wrapping and Currying

It's simple to wrap a call into an anonymous subroutine and pass that code
reference around. For example:

    $nextval_fn = sub { SP->nextval(@_) };
    ...
    $val = $nextval_fn->($sequence_name);

or

    $some_func = sub { call('some_func(int, date[], int)', @_) };
    ...
    $val = $some_func->($foo, \@dates, $debug);

You can take this approach further by specifying some of the arguments in the
anonymous subroutine so they don't all have to be provided in the call:

    $some_func = sub { call('some_func(int, date[], int)', $foo, shift, $debug) };
    ...
    $val = $some_func->(\@dates);


=head2 Results

The C<call()> function processes return values in one of four ways depending on
two criteria: single column vs. multi-column results, and list context vs scalar context.

If the results contain a single column with the same name as the function that
was called, then those values are extracted and returned directly. This makes
simple calls very simple:

    @ary = call('generate_series(int,int)', 10, 15); # (10,11,12,13,14,15)

Otherwise, the rows are returned as references to hashes:

    @rows = call('pg_get_keywords'); # ({...}, {...}, ...)

If the C<call()> function was executed in list context then all the values/rows
are returned, as shown above.

If the function was executed in scalar context then an exception will be thrown
if more than one row is returned. For example:

    $foo = call('generate_series(int,int)', 10, 10); # 10
    $bar = call('generate_series(int,int)', 10, 11); # dies

If you only want the first result you can use list context;

    ($bar) =  call('generate_series(int,int)', 10, 11);
     $bar  = (call('generate_series(int,int)', 10, 11))[0];


=head2 Performance

Internally C<call()> uses C<spi_prepare()> to create a plan to execute the
function with the typed arguments.

The plan is cached using the call 'signature' as the key. Minor variations in
the signature will still reuse the same plan.

For varadic functions, separate plans are created and cached for each distinct
number of arguments the function is called with.

=head2 Limitations and Caveats

Requires PostgreSQL 9.0 or later.

Types that contain a comma can't be used in the call signature. That's not a
problem in practice as it only affects 'C<numeric(p,s)>' and 'C<decimal(p,s)>'
and the 'C<,s>' part isn't needed. Typically the 'C<(p,s)>' portion isn't used in
signatures.

Functions with a varadic argument can't be called with no values for that
argument.  You'll get a "function ... does not exist" error. This appears to be
a PostgreSQL limitation.

The return value of functions that have a C<void> return type should not be
relied upon, naturally.

=cut

use strict;
use warnings;
use Exporter;
use Carp;

our @ISA = qw(Exporter);
our @EXPORT = qw(call SP);

my %sig_cache;
our $debug = 0;

# encapsulated package to provide an AUTOLOAD interface to call()
use constant SP => do { 
    package PostgreSQL::PLPerl::Call::SP;

    sub AUTOLOAD {
        #(my $function = our $AUTOLOAD) =~ s/.*:://;
        our $AUTOLOAD =~ s/.*:://;
        shift;
        return PostgreSQL::PLPerl::Call::call($AUTOLOAD, @_);
    }

    __PACKAGE__;
};


sub call {
    my $sig = shift;

    my $arity = scalar @_; # argument count to handle varadic subs

    my $how = $sig_cache{"$sig.$arity"} ||= do {

        # get a normalized signature to recheck the cache with
        # and also extract the SP name and argument types
        my ($stdsig, $fullspname, $spname, $arg_types) = parse_signature($sig, $arity)
            or croak "Can't parse '$sig'";
        warn "parsed call($sig) => $stdsig\n"
            if $debug;

        # recheck the cache with with the normalized signature
        $sig_cache{"$stdsig.$arity"} ||= [ # else a new entry (for both caches)
            $spname,
            scalar mk_process_args($arg_types),
            scalar mk_process_call($fullspname, $arity, $arg_types),
            $fullspname,
            $stdsig,
        ];
    };

    my ($spname, $prepargs, $callsub) = @$how;

    my $rv = $callsub->( $prepargs ? $prepargs->(@_) : @_ );

    my $rows = $rv->{rows};
    my $row1 = $rows->[0] # peek at first row
        or return;        # no row: undef in scalar context else empty list

    my $is_single_column = (keys %$row1 == 1 and exists $row1->{$spname});

    if (wantarray) {                   # list context - all rows

        return map { $_->{$spname} } @$rows if $is_single_column;
        return @$rows;
    }
    elsif (defined wantarray) {        # scalar context - single row

        croak "$sig was called in scalar context but returned more than one row"
            if @$rows > 1;

        return $row1->{$spname} if $is_single_column;
        return $row1;
    }
    # else void context - nothing to do
    return;
}


sub parse_signature {
    my ($sig, $arity) = @_;

    # extract types from signature, if any
    my $arg_types;
    if ($sig =~ s/\s*\((.*?)\)\s*$//) {
        $arg_types = [ split(/\s*,\s*/, lc($1), -1) ];
        s/^\s+// for @$arg_types;
        s/\s+$// for @$arg_types;

        # if varadic, replace '...' marker with the appropriate number
        # of copies of the preceding type name
        if (@$arg_types and $arg_types->[-1] =~ s/\s*\.\.\.//) {
            my $varadic_type = pop @$arg_types;
            push @$arg_types, $varadic_type
                until @$arg_types >= $arity;
        }
    }

    # the full name is what's left in sig
    my $fullspname = $sig;
    (my $spname = $fullspname) =~ s/.*\.//; # remove schema, if any

    # compose a normalized signature
    my $stdsig = "$fullspname".
        ($arg_types ? "(".join(",",@$arg_types).")" : "");

    return ($stdsig, $fullspname, $spname, $arg_types);
}


sub mk_process_args {
    my ($arg_types) = @_;

    return undef unless $arg_types;

    # return a closure that pre-processes the arguments of the call
    # else undef if no argument pre-processing is required

    my $hooks;
    my $i = 0;
    for my $type (@$arg_types) {
        if ($type =~ /\[/) {    # ARRAY
            $hooks->{$i} = sub { return ::encode_array_literal(shift) };
        }
        ++$i;
    }

    return undef unless $hooks;

    my $sub = sub {
        my @args = @_;
        while ( my ($argidx, $preproc) = each %$hooks ) {
            $args[$argidx] = $preproc->($args[$argidx]);
        }
        return @args;
    };

    return $sub;
}


sub mk_process_call {
    my ($spname, $arity, $arg_types) = @_;

    # return a closure that will execute the query and return result ref

    my $placeholders = join ",", map { '$'.$_ } 1..$arity;
    my $sql = "select * from $spname($placeholders)";
    my $plan = eval { ::spi_prepare($sql, $arg_types ? @$arg_types : ()) };
    if ($@) { # internal error, should never happen
        chomp $@;
        croak "$@ while preparing $sql";
    }

    my $sub = sub {
        # XXX need to catch exceptions from here and rethrow using croak
        # to appear to come from the callers location (outside this package)
        warn "calling $sql(@_) [@{$arg_types||[]}]"
            if $debug;
        return ::spi_exec_prepared($plan, @_)
    };

    return $sub;
}

1;

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