apachesolr.install clean up hook_enable(), uninstall, update

Since you delete from apachesolr_search_nodes prior to doing the insert query no need to LEFT JOIN it in and check for NULL.

Also, node_comment_stats ALWAYS as a record for each node

comment_nodeapi()...
case 'insert':
      db_query('INSERT INTO {node_comment_statistics} (nid, last_comment_timestamp, last_comment_name, last_comment_uid, comment_count) VALUES (%d, %d, NULL, %d, 0)', $node->nid, $node->changed, $node->uid);
      break;

This results in a severe improvement when enabling the module