PostgreSQL fixes; mostly, _weblinks_get_query() rewrite

Weblinks does not work for PostgreSQL anymore, at all. (After my previous patch, I was able to upgrade stuff to the latest version. But not use it :) )

It came down to redoing a lot of the code in _weblinks_get_query().

1)
The reason this started out with, is 'the standard one'. In Postgres (and other SQL92 compatible systems?) one:
- should always specify all non-aggregated fields of an aggregate query in the GROUP BY clause. (See the modification to weblinks_block.module in the patch)
- cannot do a query like 'SELECT DISCTINCT(n.x) FROM n ORDER BY n.y'. A 'distinct' query is also a kind of aggregate query and you cannot 'just' order on an unspecified field. That should also be mentioned in a GROUP BY. => it usually becomes something like 'SELECT n.x FROM n GROUP BY n.y, n.x ORDER BY n.y'.

2)
So I started out doing that, and discovered I could not just use $query. I split stuff out into $cols, $join, $where, ... to manipulate those separately, and see/test what the code was doing in all those cases, and in which cases I should add a GROUP BY clause.

3)
...and while testing, I discovered I never needed to group fields. I could just get rid of the DISTINCT altogether (because the queries would never return duplicate rows) if I just got rid of unnecessary join clauses.

I left the code block 'if ($groupby)' in just for your overview (so you can see if it ever comes in handy in the future) but it is not necessary now because $groupby is never set.

I removed the join on node_revisions fully, by the way, because it wasn't doing anything useful. And while typing this, I realise that we can also get rid of the join on 'weblinks' (except when $sort=='popular') if we just always include "n.type = 'weblinks'" in $where.

Patch is included, but I added the full changed code here for overview.
I tested this by creating some weblinks (unpublished ones, ones with zero, single and multiple categories, by multiple users). The weblinks overview page and all blocks work on mysql as well as postgres, and there is no visible 'before/after' change when testing mysql.

function _weblinks_get_query($tid = 0, $sort = 'title', $limit = 0) {
  if ($tid === 'unpublished') {
    $where = 'WHERE n.status = 0 ';
  }
  else {
    $where = 'WHERE n.status = 1 ';
  }

  $join = 'INNER JOIN {weblinks} bw ON n.vid = bw.vid '; // acts as filter on {node}
  // In some cases we need a join on term_node; in some we don't. We decide later according to $sort
  if (function_exists('taxonomy_get_term')) {
    if ($tid == 0) {
      $join_tn = 'LEFT JOIN {term_node} tn ON tn.nid=n.nid AND tn.vid=n.vid ';
      $where_tn = 'AND tn.nid IS NULL ';
    }
    else {
      $join_tn = 'INNER JOIN {term_node} tn ON tn.nid=n.nid AND tn.vid=n.vid ';
      $where_tn = 'AND tn.tid IN (%s) ';
    }
  }
  else {
    $join_tn = '';
    $where_tn = '';
  }

  $groupby = FALSE;
  $xgrpcols = '';
  switch ($sort) {
    case 'standard':
      $cols = 'n.nid, n.sticky, n.created';
      $join .= $join_tn;
      $where .= $where_tn;
//      $groupby = ($join_tn != '');
      $order = 'ORDER BY n.sticky DESC, n.created ASC';
      break;
    case 'title':
      $cols = 'n.nid, n.title';
      $join .= $join_tn;
      $where .= $where_tn;
//      $groupby = ($join_tn != '');
//      $xgrpcols = 'n.sticky'; // otherwise you can't order on it (at least not on pgsql)
      $order .= 'ORDER BY n.sticky DESC, n.title ASC';
      break;
    case 'popular':
      $cols = 'n.nid, bw.click_count, bw.last_click';
      $where .= 'AND bw.click_count>0 ';
      $order .= 'ORDER BY bw.click_count DESC, bw.last_click DESC';
      break;
    case 'user':
      $cols = 'n.nid, n.sticky, n.created';
      $where .= 'AND n.uid=%s ';
      $order .= 'ORDER BY n.sticky, n.created';
      break;
    case 'recent':
      $cols = 'n.nid, n.changed, n.title';
      if ($tid != 0) {
        $join .= $join_tn;
        $where .= $where_tn;
//        $groupby = ($join_tn != '');
      }
      $order .= 'ORDER BY n.changed DESC, n.title ASC';
      break;
    case 'random':
      $cols = 'n.nid';
      if ($tid != 0) {
        $join .= $join_tn;
        $where .= $where_tn;
      }
      $order .= 'ORDER BY RAND()';
      break;
    case 'count':
      $cols = 'COUNT(n.nid)';
      if ($tid !== 'unpublished') {
        $join .= $join_tn;
        $where .= $where_tn;
      }
      $order = '';
      break;
  }
  
  if ($groupby) {    
    $groupby = 'GROUP BY ' . $cols;
    if ($xgrpcols != '') {
      $groupby .= ', ' . $xgrpcols;
    }
    $groupby .= ' ';
  }
  else {
    $groupby = '';
  }
  
  $query = 'SELECT ' . $cols . ' FROM {node} n ' . $join . $where . $groupby . $order;
  $query = db_rewrite_sql($query);
...