[dbsrgits/DBIx-Class.git] / lib / DBIx / Class / Storage / DBIHacks.pm

package   #hide from PAUSE
  DBIx::Class::Storage::DBIHacks;

#
# This module contains code that should never have seen the light of day,
# does not belong in the Storage, or is otherwise unfit for public
# display. The arrival of SQLA2 should immediately obsolete 90% of this
#

use strict;
use warnings;

use base 'DBIx::Class::Storage';
use mro 'c3';

use List::Util 'first';
use Scalar::Util 'blessed';
use Sub::Name 'subname';
use Data::Query::Constants;
use Data::Query::ExprHelpers;
use namespace::clean;

#
# This code will remove non-selecting/non-restricting joins from
# {from} specs, aiding the RDBMS query optimizer
#
sub _prune_unused_joins {
  my ($self, $attrs) = @_;

  # only standard {from} specs are supported, and we could be disabled in general
  return ($attrs->{from}, {})  unless (
    ref $attrs->{from} eq 'ARRAY'
      and
    @{$attrs->{from}} > 1
      and
    ref $attrs->{from}[0] eq 'HASH'
      and
    ref $attrs->{from}[1] eq 'ARRAY'
      and
    $self->_use_join_optimizer
  );

  my $orig_aliastypes = $self->_resolve_aliastypes_from_select_args($attrs);

  my $new_aliastypes = { %$orig_aliastypes };

  # we will be recreating this entirely
  my @reclassify = 'joining';

  # a grouped set will not be affected by amount of rows. Thus any
  # purely multiplicator classifications can go
  # (will be reintroduced below if needed by something else)
  push @reclassify, qw(multiplying premultiplied)
    if $attrs->{_force_prune_multiplying_joins} or $attrs->{group_by};

  # nuke what will be recalculated
  delete @{$new_aliastypes}{@reclassify};

  my @newfrom = $attrs->{from}[0]; # FROM head is always present

  # recalculate what we need once the multipliers are potentially gone
  # ignore premultiplies, since they do not add any value to anything
  my %need_joins;
  for ( @{$new_aliastypes}{grep { $_ ne 'premultiplied' } keys %$new_aliastypes }) {
    # add all requested aliases
    $need_joins{$_} = 1 for keys %$_;

    # add all their parents (as per joinpath which is an AoH { table => alias })
    $need_joins{$_} = 1 for map { values %$_ } map { @{$_->{-parents}} } values %$_;
  }

  for my $j (@{$attrs->{from}}[1..$#{$attrs->{from}}]) {
    push @newfrom, $j if (
      (! defined $j->[0]{-alias}) # legacy crap
        ||
      $need_joins{$j->[0]{-alias}}
    );
  }

  # we have a new set of joiners - for everything we nuked pull the classification
  # off the original stack
  for my $ctype (@reclassify) {
    $new_aliastypes->{$ctype} = { map
      { $need_joins{$_} ? ( $_ => $orig_aliastypes->{$ctype}{$_} ) : () }
      keys %{$orig_aliastypes->{$ctype}}
    }
  }

  return ( \@newfrom, $new_aliastypes );
}

#
# This is the code producing joined subqueries like:
# SELECT me.*, other.* FROM ( SELECT me.* FROM ... ) JOIN other ON ...
#
sub _adjust_select_args_for_complex_prefetch {
  my ($self, $attrs) = @_;

  $self->throw_exception ('Complex prefetches are not supported on resultsets with a custom from attribute') unless (
    ref $attrs->{from} eq 'ARRAY'
      and
    @{$attrs->{from}} > 1
      and
    ref $attrs->{from}[0] eq 'HASH'
      and
    ref $attrs->{from}[1] eq 'ARRAY'
  );

  my $root_alias = $attrs->{alias};

  # generate inner/outer attribute lists, remove stuff that doesn't apply
  my $outer_attrs = { %$attrs };
  delete @{$outer_attrs}{qw(from bind rows offset group_by _grouped_by_distinct having)};

  my $inner_attrs = { %$attrs };
  delete @{$inner_attrs}{qw(for collapse select as _related_results_construction)};

  # there is no point of ordering the insides if there is no limit
  delete $inner_attrs->{order_by} if (
    delete $inner_attrs->{_order_is_artificial}
      or
    ! $inner_attrs->{rows}
  );

  # generate the inner/outer select lists
  # for inside we consider only stuff *not* brought in by the prefetch
  # on the outside we substitute any function for its alias
  $outer_attrs->{select} = [ @{$attrs->{select}} ];

  my ($root_node, $root_node_offset);

  for my $i (0 .. $#{$inner_attrs->{from}}) {
    my $node = $inner_attrs->{from}[$i];
    my $h = (ref $node eq 'HASH')                                ? $node
          : (ref $node  eq 'ARRAY' and ref $node->[0] eq 'HASH') ? $node->[0]
          : next
    ;

    if ( ($h->{-alias}||'') eq $root_alias and $h->{-rsrc} ) {
      $root_node = $h;
      $root_node_offset = $i;
      last;
    }
  }

  $self->throw_exception ('Complex prefetches are not supported on resultsets with a custom from attribute')
    unless $root_node;

  # use the heavy duty resolver to take care of aliased/nonaliased naming
  my $colinfo = $self->_resolve_column_info($inner_attrs->{from});
  my $selected_root_columns;

  for my $i (0 .. $#{$outer_attrs->{select}}) {
    my $sel = $outer_attrs->{select}->[$i];

    next if (
      $colinfo->{$sel} and $colinfo->{$sel}{-source_alias} ne $root_alias
    );

    if (ref $sel eq 'HASH' ) {
      $sel->{-as} ||= $attrs->{as}[$i];
      $outer_attrs->{select}->[$i] = join ('.', $root_alias, ($sel->{-as} || "inner_column_$i") );
    }
    elsif (! ref $sel and my $ci = $colinfo->{$sel}) {
      $selected_root_columns->{$ci->{-colname}} = 1;
    }

    push @{$inner_attrs->{select}}, $sel;

    push @{$inner_attrs->{as}}, $attrs->{as}[$i];
  }

  # We will need to fetch all native columns in the inner subquery, which may
  # be a part of an *outer* join condition, or an order_by (which needs to be
  # preserved outside), or wheres. In other words everything but the inner
  # selector
  # We can not just fetch everything because a potential has_many restricting
  # join collapse *will not work* on heavy data types.
  my $connecting_aliastypes = $self->_resolve_aliastypes_from_select_args({
    %$inner_attrs,
    select => undef,
  });

  for (sort map { keys %{$_->{-seen_columns}||{}} } map { values %$_ } values %$connecting_aliastypes) {
    my $ci = $colinfo->{$_} or next;
    if (
      $ci->{-source_alias} eq $root_alias
        and
      ! $selected_root_columns->{$ci->{-colname}}++
    ) {
      # adding it to both to keep limits not supporting dark selectors happy
      push @{$inner_attrs->{select}}, $ci->{-fq_colname};
      push @{$inner_attrs->{as}}, $ci->{-fq_colname};
    }
  }

  # construct the inner {from} and lock it in a subquery
  # we need to prune first, because this will determine if we need a group_by below
  # throw away all non-selecting, non-restricting multijoins
  # (since we def. do not care about multiplication of the contents of the subquery)
  my $inner_subq = do {

    # must use it here regardless of user requests (vastly gentler on optimizer)
    local $self->{_use_join_optimizer} = 1;

    # throw away multijoins since we def. do not care about those inside the subquery
    ($inner_attrs->{from}, my $inner_aliastypes) = $self->_prune_unused_joins ({
      %$inner_attrs, _force_prune_multiplying_joins => 1
    });

    # uh-oh a multiplier (which is not us) left in, this is a problem for limits
    # we will need to add a group_by to collapse the resultset for proper counts
    if (
      grep { $_ ne $root_alias } keys %{ $inner_aliastypes->{multiplying} || {} }
        and
      # if there are user-supplied groups - assume user knows wtf they are up to
      ( ! $inner_aliastypes->{grouping} or $inner_attrs->{_grouped_by_distinct} )
    ) {

      my $cur_sel = { map { $_ => 1 } @{$inner_attrs->{select}} };

      # *possibly* supplement the main selection with pks if not already
      # there, as they will have to be a part of the group_by to collapse
      # things properly
      my $inner_select_with_extras;
      my @pks = map { "$root_alias.$_" } $root_node->{-rsrc}->primary_columns
        or $self->throw_exception( sprintf
          'Unable to perform complex limited prefetch off %s without declared primary key',
          $root_node->{-rsrc}->source_name,
        );
      for my $col (@pks) {
        push @{ $inner_select_with_extras ||= [ @{$inner_attrs->{select}} ] }, $col
          unless $cur_sel->{$col}++;
      }

      ($inner_attrs->{group_by}, $inner_attrs->{order_by}) = $self->_group_over_selection({
        %$inner_attrs,
        $inner_select_with_extras ? ( select => $inner_select_with_extras ) : (),
        _aliastypes => $inner_aliastypes,
      });
    }

    # we already optimized $inner_attrs->{from} above
    # and already local()ized
    $self->{_use_join_optimizer} = 0;

    # generate the subquery
    $self->_select_args_to_query (
      @{$inner_attrs}{qw(from select where)},
      $inner_attrs,
    );
  };

  # Generate the outer from - this is relatively easy (really just replace
  # the join slot with the subquery), with a major caveat - we can not
  # join anything that is non-selecting (not part of the prefetch), but at
  # the same time is a multi-type relationship, as it will explode the result.
  #
  # There are two possibilities here
  # - either the join is non-restricting, in which case we simply throw it away
  # - it is part of the restrictions, in which case we need to collapse the outer
  #   result by tackling yet another group_by to the outside of the query

  # work on a shallow copy
  my @orig_from = @{$attrs->{from}};


  $outer_attrs->{from} = \ my @outer_from;

  # we may not be the head
  if ($root_node_offset) {
    # first generate the outer_from, up to the substitution point
    @outer_from = splice @orig_from, 0, $root_node_offset;

    # substitute the subq at the right spot
    push @outer_from, [
      {
        -alias => $root_alias,
        -rsrc => $root_node->{-rsrc},
        $root_alias => $inner_subq,
      },
      # preserve attrs from what is now the head of the from after the splice
      @{$orig_from[0]}[1 .. $#{$orig_from[0]}],
    ];
  }
  else {
    @outer_from = {
      -alias => $root_alias,
      -rsrc => $root_node->{-rsrc},
      $root_alias => $inner_subq,
    };
  }

  shift @orig_from; # what we just replaced above

  # scan the *remaining* from spec against different attributes, and see which joins are needed
  # in what role
  my $outer_aliastypes = $outer_attrs->{_aliastypes} =
    $self->_resolve_aliastypes_from_select_args({ %$outer_attrs, from => \@orig_from });

  # unroll parents
  my ($outer_select_chain, @outer_nonselecting_chains) = map { +{
    map { $_ => 1 } map { values %$_} map { @{$_->{-parents}} } values %{ $outer_aliastypes->{$_} || {} }
  } } qw/selecting restricting grouping ordering/;

  # see what's left - throw away if not selecting/restricting
  my $may_need_outer_group_by;
  while (my $j = shift @orig_from) {
    my $alias = $j->[0]{-alias};

    if (
      $outer_select_chain->{$alias}
    ) {
      push @outer_from, $j
    }
    elsif (first { $_->{$alias} } @outer_nonselecting_chains ) {
      push @outer_from, $j;
      $may_need_outer_group_by ||= $outer_aliastypes->{multiplying}{$alias} ? 1 : 0;
    }
  }

  # also throw in a synthetic group_by if a non-selecting multiplier,
  # to guard against cross-join explosions
  # the logic is somewhat fragile, but relies on the idea that if a user supplied
  # a group by on their own - they know what they were doing
  if ( $may_need_outer_group_by and $attrs->{_grouped_by_distinct} ) {
    ($outer_attrs->{group_by}, $outer_attrs->{order_by}) = $self->_group_over_selection ({
      %$outer_attrs,
      from => \@outer_from,
    });
  }

  # This is totally horrific - the {where} ends up in both the inner and outer query
  # Unfortunately not much can be done until SQLA2 introspection arrives, and even
  # then if where conditions apply to the *right* side of the prefetch, you may have
  # to both filter the inner select (e.g. to apply a limit) and then have to re-filter
  # the outer select to exclude joins you didn't want in the first place
  #
  # OTOH it can be seen as a plus: <ash> (notes that this query would make a DBA cry ;)
  return $outer_attrs;
}

#
# I KNOW THIS SUCKS! GET SQLA2 OUT THE DOOR SO THIS CAN DIE!
#
# Due to a lack of SQLA2 we fall back to crude scans of all the
# select/where/order/group attributes, in order to determine what
# aliases are needed to fulfill the query. This information is used
# throughout the code to prune unnecessary JOINs from the queries
# in an attempt to reduce the execution time.
# Although the method is pretty horrific, the worst thing that can
# happen is for it to fail due to some scalar SQL, which in turn will
# result in a vocal exception.
sub _resolve_aliastypes_from_select_args {
  my ( $self, $attrs ) = @_;

  $self->throw_exception ('Unable to analyze custom {from}')
    if ref $attrs->{from} ne 'ARRAY';

  # what we will return
  my $aliases_by_type;

  # see what aliases are there to work with
  # and record who is a multiplier and who is premultiplied
  my $alias_list;
  for my $node (@{$attrs->{from}}) {

    my $j = $node;
    $j = $j->[0] if ref $j eq 'ARRAY';
    my $al = $j->{-alias}
      or next;

    $alias_list->{$al} = $j;

    $aliases_by_type->{multiplying}{$al} ||= { -parents => $j->{-join_path}||[] }
      # not array == {from} head == can't be multiplying
      if ref($node) eq 'ARRAY' and ! $j->{-is_single};

    $aliases_by_type->{premultiplied}{$al} ||= { -parents => $j->{-join_path}||[] }
      # parts of the path that are not us but are multiplying
      if grep { $aliases_by_type->{multiplying}{$_} }
          grep { $_ ne $al }
           map { values %$_ }
            @{ $j->{-join_path}||[] }
  }

  # get a column to source/alias map (including unambiguous unqualified ones)
  my $colinfo = $self->_resolve_column_info ($attrs->{from});

  # set up a botched SQLA
  my $sql_maker = $self->sql_maker;

  # these are throw away results, do not pollute the bind stack
  local $sql_maker->{select_bind};
  local $sql_maker->{where_bind};
  local $sql_maker->{group_bind};
  local $sql_maker->{having_bind};
  local $sql_maker->{from_bind};

  # we can't scan properly without any quoting (\b doesn't cut it
  # everywhere), so unless there is proper quoting set - use our
  # own weird impossible character.
  # Also in the case of no quoting, we need to explicitly disable
  # name_sep, otherwise sorry nasty legacy syntax like
  # { 'count(foo.id)' => { '>' => 3 } } will stop working >:(
  local $sql_maker->{quote_char} = $sql_maker->{quote_char};
  local $sql_maker->{name_sep} = $sql_maker->{name_sep};

  unless (defined $sql_maker->{quote_char} and length $sql_maker->{quote_char}) {
    $sql_maker->{quote_char} = ["\x00", "\xFF"];
    # if we don't unset it we screw up retarded but unfortunately working
    # 'MAX(foo.bar)' => { '>', 3 }
    $sql_maker->{name_sep} = '';
  }

  # delete local is 5.12+
  local @{$sql_maker}{qw(renderer converter)};
  delete @{$sql_maker}{qw(renderer converter)};

  my ($lquote, $rquote, $sep) = map { quotemeta $_ } ($sql_maker->_quote_chars, $sql_maker->name_sep);

  # generate sql chunks
  my $to_scan = {
    restricting => [
      ($attrs->{where}
        ? ($sql_maker->_recurse_where($attrs->{where}))[0]
        : ()
      ),
      ($attrs->{having}
        ? ($sql_maker->_recurse_where($attrs->{having}))[0]
        : ()
      ),
    ],
    grouping => [
      ($attrs->{group_by}
        ? ($sql_maker->_render_sqla(group_by => $attrs->{group_by}))[0]
        : (),
      )
    ],
    joining => [
      $sql_maker->_recurse_from (
        ref $attrs->{from}[0] eq 'ARRAY' ? $attrs->{from}[0][0] : $attrs->{from}[0],
        @{$attrs->{from}}[1 .. $#{$attrs->{from}}],
      ),
    ],
    selecting => [
      ($attrs->{select}
        ? ($sql_maker->_render_sqla(select_select => $attrs->{select}))[0]
        : ()),
    ],
    ordering => [
      map { $_->[0] } $self->_extract_order_criteria ($attrs->{order_by}, $sql_maker),
    ],
  };

  # throw away empty chunks
  $_ = [ map { $_ || () } @$_ ] for values %$to_scan;

  # first see if we have any exact matches (qualified or unqualified)
  for my $type (keys %$to_scan) {
    for my $piece (@{$to_scan->{$type}}) {
      if ($colinfo->{$piece} and my $alias = $colinfo->{$piece}{-source_alias}) {
        $aliases_by_type->{$type}{$alias} ||= { -parents => $alias_list->{$alias}{-join_path}||[] };
        $aliases_by_type->{$type}{$alias}{-seen_columns}{$colinfo->{$piece}{-fq_colname}} = $piece;
      }
    }
  }

  # now loop through all fully qualified columns and get the corresponding
  # alias (should work even if they are in scalarrefs)
  for my $alias (keys %$alias_list) {
    my $al_re = qr/
      $lquote $alias $rquote $sep (?: $lquote ([^$rquote]+) $rquote )?
        |
      \b $alias \. ([^\s\)\($rquote]+)?
    /x;

    for my $type (keys %$to_scan) {
      for my $piece (@{$to_scan->{$type}}) {
        if (my @matches = $piece =~ /$al_re/g) {
          $aliases_by_type->{$type}{$alias} ||= { -parents => $alias_list->{$alias}{-join_path}||[] };
          $aliases_by_type->{$type}{$alias}{-seen_columns}{"$alias.$_"} = "$alias.$_"
            for grep { defined $_ } @matches;
        }
      }
    }
  }

  # now loop through unqualified column names, and try to locate them within
  # the chunks
  for my $col (keys %$colinfo) {
    next if $col =~ / \. /x;   # if column is qualified it was caught by the above

    my $col_re = qr/ $lquote ($col) $rquote /x;

    for my $type (keys %$to_scan) {
      for my $piece (@{$to_scan->{$type}}) {
        if ( my @matches = $piece =~ /$col_re/g) {
          my $alias = $colinfo->{$col}{-source_alias};
          $aliases_by_type->{$type}{$alias} ||= { -parents => $alias_list->{$alias}{-join_path}||[] };
          $aliases_by_type->{$type}{$alias}{-seen_columns}{"$alias.$_"} = $_
            for grep { defined $_ } @matches;
        }
      }
    }
  }

  # Add any non-left joins to the restriction list (such joins are indeed restrictions)
  for my $j (values %$alias_list) {
    my $alias = $j->{-alias} or next;
    $aliases_by_type->{restricting}{$alias} ||= { -parents => $j->{-join_path}||[] } if (
      (not $j->{-join_type})
        or
      ($j->{-join_type} !~ /^left (?: \s+ outer)? $/xi)
    );
  }

  for (keys %$aliases_by_type) {
    delete $aliases_by_type->{$_} unless keys %{$aliases_by_type->{$_}};
  }

  return $aliases_by_type;
}

# This is the engine behind { distinct => 1 } and the general
# complex prefetch grouper
sub _group_over_selection {
  my ($self, $attrs) = @_;

  my $colinfos = $self->_resolve_column_info ($attrs->{from});

  my (@group_by, %group_index);

  # the logic is: if it is a { func => val } we assume an aggregate,
  # otherwise if \'...' or \[...] we assume the user knows what is
  # going on thus group over it
  for (@{$attrs->{select}}) {
    if (! ref($_) or ref ($_) ne 'HASH' ) {
      push @group_by, $_;
      $group_index{$_}++;
      if ($colinfos->{$_} and $_ !~ /\./ ) {
        # add a fully qualified version as well
        $group_index{"$colinfos->{$_}{-source_alias}.$_"}++;
      }
    }
  }

  my $sql_maker = $self->sql_maker;
  my @order_by = $self->_extract_order_criteria($attrs->{order_by}, $sql_maker)
    or return (\@group_by, $attrs->{order_by});

  # add any order_by parts that are not already present in the group_by
  # to maintain SQL cross-compatibility and general sanity
  #
  # also in case the original selection is *not* unique, or in case part
  # of the ORDER BY refers to a multiplier - we will need to replace the
  # skipped order_by elements with their MIN/MAX equivalents as to maintain
  # the proper overall order without polluting the group criteria (and
  # possibly changing the outcome entirely)

  my ($leftovers, @new_order_by, $order_chunks, $aliastypes);

  my $group_already_unique = $self->_columns_comprise_identifying_set($colinfos, \@group_by);

  for my $o_idx (0 .. $#order_by) {

    # if the chunk is already a min/max function - there is nothing left to touch
    next if $order_by[$o_idx][0] =~ /^ (?: min | max ) \s* \( .+ \) $/ix;

    # only consider real columns (for functions the user got to do an explicit group_by)
    my $chunk_ci;
    if (
      @{$order_by[$o_idx]} != 1
        or
      # only declare an unknown *plain* identifier as "leftover" if we are called with
      # aliastypes to examine. If there are none - we are still in _resolve_attrs, and
      # can just assume the user knows what they want
      ( ! ( $chunk_ci = $colinfos->{$order_by[$o_idx][0]} ) and $attrs->{_aliastypes} )
    ) {
      push @$leftovers, $order_by[$o_idx][0];
    }

    next unless $chunk_ci;

    # no duplication of group criteria
    next if $group_index{$chunk_ci->{-fq_colname}};

    $aliastypes ||= (
      $attrs->{_aliastypes}
        or
      $self->_resolve_aliastypes_from_select_args({
        from => $attrs->{from},
        order_by => $attrs->{order_by},
      })
    ) if $group_already_unique;

    # check that we are not ordering by a multiplier (if a check is requested at all)
    if (
      $group_already_unique
        and
      ! $aliastypes->{multiplying}{$chunk_ci->{-source_alias}}
        and
      ! $aliastypes->{premultiplied}{$chunk_ci->{-source_alias}}
    ) {
      push @group_by, $chunk_ci->{-fq_colname};
      $group_index{$chunk_ci->{-fq_colname}}++
    }
    else {
      # We need to order by external columns without adding them to the group
      # (eiehter a non-unique selection, or a multi-external)
      #
      # This doesn't really make sense in SQL, however from DBICs point
      # of view is rather valid (e.g. order the leftmost objects by whatever
      # criteria and get the offset/rows many). There is a way around
      # this however in SQL - we simply tae the direction of each piece
      # of the external order and convert them to MIN(X) for ASC or MAX(X)
      # for DESC, and group_by the root columns. The end result should be
      # exactly what we expect

      # FIXME - this code is a joke, will need to be completely rewritten in
      # the DQ branch. But I need to push a POC here, otherwise the
      # pesky tests won't pass
      # wrap any part of the order_by that "responds" to an ordering alias
      # into a MIN/MAX

      $order_chunks ||= do {
        my @c;
        my $dq_node = $sql_maker->converter->_order_by_to_dq($attrs->{order_by});

        while (is_Order($dq_node)) {
          push @c, {
            is_desc => $dq_node->{reverse},
            dq_node => $dq_node->{by},
          };

          @{$c[-1]}{qw(sql bind)} = $sql_maker->_render_dq($dq_node->{by});

          $dq_node = $dq_node->{from};
        }

        \@c;
      };

      $new_order_by[$o_idx] = {
        ($order_chunks->[$o_idx]{is_desc} ? '-desc' : '-asc') => \[
          sprintf ( '%s( %s )',
            ($order_chunks->[$o_idx]{is_desc} ? 'MAX' : 'MIN'),
            $order_chunks->[$o_idx]{sql},
          ),
          @{ $order_chunks->[$o_idx]{bind} || [] }
        ]
      };
    }
  }

  $self->throw_exception ( sprintf
    'A required group_by clause could not be constructed automatically due to a complex '
  . 'order_by criteria (%s). Either order_by columns only (no functions) or construct a suitable '
  . 'group_by by hand',
    join ', ', map { "'$_'" } @$leftovers,
  ) if $leftovers;

  # recreate the untouched order parts
  if (@new_order_by) {
    $new_order_by[$_] ||= {
      ( $order_chunks->[$_]{is_desc} ? '-desc' : '-asc' )
        => \ $order_chunks->[$_]{dq_node}
    } for ( 0 .. $#$order_chunks );
  }

  return (
    \@group_by,
    (@new_order_by ? \@new_order_by : $attrs->{order_by} ),  # same ref as original == unchanged
  );
}

sub _resolve_ident_sources {
  my ($self, $ident) = @_;

  my $alias2source = {};

  # the reason this is so contrived is that $ident may be a {from}
  # structure, specifying multiple tables to join
  if ( blessed $ident && $ident->isa("DBIx::Class::ResultSource") ) {
    # this is compat mode for insert/update/delete which do not deal with aliases
    $alias2source->{me} = $ident;
  }
  elsif (ref $ident eq 'ARRAY') {

    for (@$ident) {
      my $tabinfo;
      if (ref $_ eq 'HASH') {
        $tabinfo = $_;
      }
      if (ref $_ eq 'ARRAY' and ref $_->[0] eq 'HASH') {
        $tabinfo = $_->[0];
      }

      $alias2source->{$tabinfo->{-alias}} = $tabinfo->{-rsrc}
        if ($tabinfo->{-rsrc});
    }
  }

  return $alias2source;
}

# Takes $ident, \@column_names
#
# returns { $column_name => \%column_info, ... }
# also note: this adds -result_source => $rsrc to the column info
#
# If no columns_names are supplied returns info about *all* columns
# for all sources
sub _resolve_column_info {
  my ($self, $ident, $colnames) = @_;
  my $alias2src = $self->_resolve_ident_sources($ident);

  my (%seen_cols, @auto_colnames);

  # compile a global list of column names, to be able to properly
  # disambiguate unqualified column names (if at all possible)
  for my $alias (keys %$alias2src) {
    my $rsrc = $alias2src->{$alias};
    for my $colname ($rsrc->columns) {
      push @{$seen_cols{$colname}}, $alias;
      push @auto_colnames, "$alias.$colname" unless $colnames;
    }
  }

  $colnames ||= [
    @auto_colnames,
    grep { @{$seen_cols{$_}} == 1 } (keys %seen_cols),
  ];

  my (%return, $colinfos);
  foreach my $col (@$colnames) {
    my ($source_alias, $colname) = $col =~ m/^ (?: ([^\.]+) \. )? (.+) $/x;

    # if the column was seen exactly once - we know which rsrc it came from
    $source_alias ||= $seen_cols{$colname}[0]
      if ($seen_cols{$colname} and @{$seen_cols{$colname}} == 1);

    next unless $source_alias;

    my $rsrc = $alias2src->{$source_alias}
      or next;

    $return{$col} = {
      %{
          ( $colinfos->{$source_alias} ||= $rsrc->columns_info )->{$colname}
            ||
          $self->throw_exception(
            "No such column '$colname' on source " . $rsrc->source_name
          );
      },
      -result_source => $rsrc,
      -source_alias => $source_alias,
      -fq_colname => $col eq $colname ? "$source_alias.$col" : $col,
      -colname => $colname,
    };

    $return{"$source_alias.$colname"} = $return{$col} if $col eq $colname;
  }

  return \%return;
}

# The DBIC relationship chaining implementation is pretty simple - every
# new related_relationship is pushed onto the {from} stack, and the {select}
# window simply slides further in. This means that when we count somewhere
# in the middle, we got to make sure that everything in the join chain is an
# actual inner join, otherwise the count will come back with unpredictable
# results (a resultset may be generated with _some_ rows regardless of if
# the relation which the $rs currently selects has rows or not). E.g.
# $artist_rs->cds->count - normally generates:
# SELECT COUNT( * ) FROM artist me LEFT JOIN cd cds ON cds.artist = me.artistid
# which actually returns the number of artists * (number of cds || 1)
#
# So what we do here is crawl {from}, determine if the current alias is at
# the top of the stack, and if not - make sure the chain is inner-joined down
# to the root.
#
sub _inner_join_to_node {
  my ($self, $from, $alias) = @_;

  # subqueries and other oddness are naturally not supported
  return $from if (
    ref $from ne 'ARRAY'
      ||
    @$from <= 1
      ||
    ref $from->[0] ne 'HASH'
      ||
    ! $from->[0]{-alias}
      ||
    $from->[0]{-alias} eq $alias  # this last bit means $alias is the head of $from - nothing to do
  );

  # find the current $alias in the $from structure
  my $switch_branch;
  JOINSCAN:
  for my $j (@{$from}[1 .. $#$from]) {
    if ($j->[0]{-alias} eq $alias) {
      $switch_branch = $j->[0]{-join_path};
      last JOINSCAN;
    }
  }

  # something else went quite wrong
  return $from unless $switch_branch;

  # So it looks like we will have to switch some stuff around.
  # local() is useless here as we will be leaving the scope
  # anyway, and deep cloning is just too fucking expensive
  # So replace the first hashref in the node arrayref manually
  my @new_from = ($from->[0]);
  my $sw_idx = { map { (values %$_), 1 } @$switch_branch }; #there's one k/v per join-path

  for my $j (@{$from}[1 .. $#$from]) {
    my $jalias = $j->[0]{-alias};

    if ($sw_idx->{$jalias}) {
      my %attrs = %{$j->[0]};
      delete $attrs{-join_type};
      push @new_from, [
        \%attrs,
        @{$j}[ 1 .. $#$j ],
      ];
    }
    else {
      push @new_from, $j;
    }
  }

  return \@new_from;
}

sub _extract_order_criteria {
  my ($self, $order_by, $sql_maker, $ident_only) = @_;

  $sql_maker ||= $self->sql_maker;

  my $order_dq = $sql_maker->converter->_order_by_to_dq($order_by);

  my @by;
  while (is_Order($order_dq)) {
    push @by, $order_dq->{by};
    $order_dq = $order_dq->{from};
  }

  # delete local is 5.12+
  local @{$sql_maker}{qw(quote_char renderer converter)};
  delete @{$sql_maker}{qw(quote_char renderer converter)};

  return map { [ $sql_maker->_render_dq($_) ] } do {
    if ($ident_only) {
      my @by_ident;
      scan_dq_nodes({ DQ_IDENTIFIER ,=> sub { push @by_ident, $_[0] } }, @by);
      @by_ident
    } else {
      @by
    }
  };
}

sub _order_by_is_stable {
  my ($self, $ident, $order_by, $where) = @_;

  my @cols = (
    (map { $_->[0] } $self->_extract_order_criteria($order_by, undef, 1)),
    $where ? @{$self->_extract_fixed_condition_columns($where)} :(),
  ) or return undef;

  my $colinfo = $self->_resolve_column_info($ident, \@cols);

  return keys %$colinfo
    ? $self->_columns_comprise_identifying_set( $colinfo,  \@cols )
    : undef
  ;
}

sub _columns_comprise_identifying_set {
  my ($self, $colinfo, $columns) = @_;

  my $cols_per_src;
  $cols_per_src -> {$_->{-source_alias}} -> {$_->{-colname}} = $_
    for grep { defined $_ } @{$colinfo}{@$columns};

  for (values %$cols_per_src) {
    my $src = (values %$_)[0]->{-result_source};
    return 1 if $src->_identifying_column_set($_);
  }

  return undef;
}

# this is almost identical to the above, except it accepts only
# a single rsrc, and will succeed only if the first portion of the order
# by is stable.
# returns that portion as a colinfo hashref on success
sub _main_source_order_by_portion_is_stable {
  my ($self, $main_rsrc, $order_by, $where) = @_;

  die "Huh... I expect a blessed result_source..."
    if ref($main_rsrc) eq 'ARRAY';

  my @ord_cols = map
    { $_->[0] }
    ( $self->_extract_order_criteria($order_by) )
  ;
  return unless @ord_cols;

  my $colinfos = $self->_resolve_column_info($main_rsrc);

  for (0 .. $#ord_cols) {
    if (
      ! $colinfos->{$ord_cols[$_]}
        or
      $colinfos->{$ord_cols[$_]}{-result_source} != $main_rsrc
    ) {
      $#ord_cols =  $_ - 1;
      last;
    }
  }

  # we just truncated it above
  return unless @ord_cols;

  my $order_portion_ci = { map {
    $colinfos->{$_}{-colname} => $colinfos->{$_},
    $colinfos->{$_}{-fq_colname} => $colinfos->{$_},
  } @ord_cols };

  # since all we check here are the start of the order_by belonging to the
  # top level $rsrc, a present identifying set will mean that the resultset
  # is ordered by its leftmost table in a stable manner
  #
  # RV of _identifying_column_set contains unqualified names only
  my $unqualified_idset = $main_rsrc->_identifying_column_set({
    ( $where ? %{
      $self->_resolve_column_info(
        $main_rsrc, $self->_extract_fixed_condition_columns($where)
      )
    } : () ),
    %$order_portion_ci
  }) or return;

  my $ret_info;
  my %unqualified_idcols_from_order = map {
    $order_portion_ci->{$_} ? ( $_ => $order_portion_ci->{$_} ) : ()
  } @$unqualified_idset;

  # extra optimization - cut the order_by at the end of the identifying set
  # (just in case the user was stupid and overlooked the obvious)
  for my $i (0 .. $#ord_cols) {
    my $col = $ord_cols[$i];
    my $unqualified_colname = $order_portion_ci->{$col}{-colname};
    $ret_info->{$col} = { %{$order_portion_ci->{$col}}, -idx_in_order_subset => $i };
    delete $unqualified_idcols_from_order{$ret_info->{$col}{-colname}};

    # we didn't reach the end of the identifying portion yet
    return $ret_info unless keys %unqualified_idcols_from_order;
  }

  die 'How did we get here...';
}

# returns an arrayref of column names which *definitely* have some
# sort of non-nullable equality requested in the given condition
# specification. This is used to figure out if a resultset is
# constrained to a column which is part of a unique constraint,
# which in turn allows us to better predict how ordering will behave
# etc.
#
# this is a rudimentary, incomplete, and error-prone extractor
# however this is OK - it is conservative, and if we can not find
# something that is in fact there - the stack will recover gracefully
# Also - DQ and the mst it rode in on will save us all RSN!!!
sub _extract_fixed_condition_columns {
  my ($self, $where) = @_;

  if (ref($where) eq 'REF' and ref($$where) eq 'HASH') {
    # Yes. I know.
    my $fixed = DBIx::Class::ResultSource->_extract_fixed_values_for($$where);
    return [ keys %$fixed ];
  }

  return unless ref $where eq 'HASH';

  my @cols;
  for my $lhs (keys %$where) {
    if ($lhs =~ /^\-and$/i) {
      push @cols, ref $where->{$lhs} eq 'ARRAY'
        ? ( map { @{ $self->_extract_fixed_condition_columns($_) } } @{$where->{$lhs}} )
        : @{ $self->_extract_fixed_condition_columns($where->{$lhs}) }
      ;
    }
    elsif ($lhs !~ /^\-/) {
      my $val = $where->{$lhs};

      push @cols, $lhs if (defined $val and (
        ! ref $val
          or
        (ref $val eq 'HASH' and keys %$val == 1 and defined $val->{'='})
      ));
    }
  }
  return \@cols;
}

1;