12.7.SQL Parse Tree

The SQL parse tree is composed of DV_ARRAY_OF_POINTER boxes. Other types of boxes may occur as leaves, where they are interpreted as literals. All nodes' first element (index 0) is the type of the node, one of the constants in sqlparext.h.

The nodes' various fields can be accessed through the data members of the sql_tree_t union. Most data members are pointers to the same type. Sometimes they are double pointers, denoting a variable length array of pointers to the struct. The caddr_t type is used to denote a terminal, like a string or other constant. The types are only for declarative value, the entire structure is a self describing, run time typed tree of boxes.

The correspondence of the tree to the SQL syntax is documented by the yacc grammar supplied as appendix.

We will next examine the need to know example:

ST *
nk_tree_and (ST* left, ST * right)
  if (left && right)
    return ((ST*) list (4, BOP_AND, left, right, NULL));
  if (left)
    return left;
  return right;

This function adds anAND operation between 2 sub trees. If either is NULL, the non-null one is returned. The list function is used as a universal constructor of the parse tree, where the first argument is the count of arguments to follow.

The above could have been written as follows without list:

ST * r = (ST*) dk_alloc_box (4 * sizeof (caddr_t), DV_ARRAY_OF_POINTER);
r->type = BOP_AND;
r->_.bin_exp.left = left;
r->_.bin_exp.right = right;
r->_.bin_exp.more = NULL;

We see that the list notation is more concise.

nk_test_add (ST * outer_texp, char * corr_name, int uid)
  /* add a exists (select 1 from need_to_know where nk_class = <corr_name>.r_class) */
  ST * sel, * exists, * texp, **from;
  ST * where = (ST*) list (4, BOP_EQ, list (3, COL_DOTTED, NULL, box_string ("NK_CLASS")),
                           list (3, COL_DOTTED, box_string (corr_name), box_string ("R_CLASS")), NULL);
  where = nk_tree_and (where, listst (4, BOP_EQ, list (3, COL_DOTTED, NULL, box_string ("NK_USER")), box_num (uid), NULL));
  from = (ST**)
    list (1, list (3, TABLE_REF,
                   list (5, TABLE_DOTTED,
                         box_string ("DB.DBA.NEED_TO_KNOW"),
                         NULL, box_num (0), box_num (0)),
  texp = listst (7, TABLE_EXP, from,
               where, NULL, NULL, NULL, -1);
  sel = listst (5, SELECT_STMT, NULL, list (1, box_num (1)), NULL, texp);
  exists = (ST*) list (5, EXISTS_PRED, NULL, sel, NULL, NULL);
  outer_texp->_.table_exp.where = nk_tree_and (outer_texp->_.table_exp.where, exists);

This function takes the table expression referencing the REPORT table and the correlation name used for the table and the user id of the querying user. It adds the existence test for the need to know. The logic is self evident when examined in the context of the yacc grammar. Note that the table in the FROM of the sub query is a TABLE_REF node with a TABLE_DOTTED node, which finally contains the table and correlation names. Note that all strings must be allocated as string boxes. This is because the tree may only reference other boxes. Note that all numbers except for the types of the nodes are boxed with box_num. This will make sure that numbers and pointers are always distinguishable. The node types are distinguishable by definition due to their small absolute value.

bif_need_to_know (caddr_t * qst, caddr_t * err_ret, state_slot_t ** args)
  unsigned inx;
  caddr_t uid = (caddr_t) bif_long_arg (qst, args, 0, "need_to_know");
  ST * tree = (ST*) bif_array_arg (qst, args, 1, "need_to_know");
  if (ST_P (tree, SELECT_STMT))
      ST * texp = tree->_.select_stmt.table_exp;
      if (!texp)
        return 0; /* select w/o a from */
      for (inx = 0; inx < BOX_ELEMENTS (texp->_.table_exp.from); inx++)
          ST * tref = texp->_.table_exp.from[inx];
          if (ST_P (tref, TABLE_REF))
            tref = tref->_.table_ref.table;
          if (ST_P (tref, TABLE_DOTTED))
              char * corr_name;
              if (tref->_.table.prefix)
                corr_name = tref->_.table.prefix;
                corr_name = tref->_.table.name;
              if (strstr (tref->_.table.name, "REPORT"))
                nk_test_add (texp, corr_name, (int)uid);
  return 0;

This is the top level need to know function. It first checks to see that the statement is a select, that it has a FROM clause (table expression). For each table in the FROM which has REPORT as part of its name the function adds an existence test to check the user's need to know. Note that the correlation name is taken from the table and that the table name is used in the absence of a correlation name to disambiguate the reference. Note that the table_exp.from is of type ST **, meaning a variable length array of boxes. Note that BOX_ELEMENTS returns the length in pointer-size units.

Note that in this example the tree is spliced in place, only adding nodes. There is no need to free data or to modify the top node.