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17.6. Creating the Operators and Support Routines

So now we have an access method and an operator class. We still need a set of operators. The procedure for defining operators was discussed in Chapter 14. For the complex_abs_ops operator class on B-trees, the operators we require are:

  • absolute-value less-than (strategy 1)
  • absolute-value less-than-or-equal (strategy 2)
  • absolute-value equal (strategy 3)
  • absolute-value greater-than-or-equal (strategy 4)
  • absolute-value greater-than (strategy 5)

Suppose the code that implements these functions is stored in the file PGROOT/src/tutorial/complex.c, which we have compiled into PGROOT/src/tutorial/complex.so. Part of the C code looks like this:

#define Mag(c) ((c)->x*(c)->x + (c)->y*(c)->y)

         bool
         complex_abs_eq(Complex *a, Complex *b)
         {
             double amag = Mag(a), bmag = Mag(b);
             return (amag==bmag);
         }

(Note that we will only show the equality operator for the rest of the examples. The other four operators are very similar. Refer to complex.c or complex.source for the details.)

We make the function known to PostgreSQL like this:

CREATE FUNCTION complex_abs_eq(complex, complex) RETURNS boolean
    AS 'PGROOT/src/tutorial/complex'
    LANGUAGE C;

There are some important things that are happening here:

  • First, note that operators for less-than, less-than-or-equal, equal, greater-than-or-equal, and greater-than for complex are being defined. We can only have one operator named, say, = and taking type complex for both operands. In this case we don't have any other operator = for complex, but if we were building a practical data type we'd probably want = to be the ordinary equality operation for complex numbers. In that case, we'd need to use some other operator name for complex_abs_eq.

  • Second, although PostgreSQL can cope with operators having the same name as long as they have different input data types, C can only cope with one global routine having a given name, period. So we shouldn't name the C function something simple like abs_eq. Usually it's a good practice to include the data type name in the C function name, so as not to conflict with functions for other data types.

  • Third, we could have made the PostgreSQL name of the function abs_eq, relying on PostgreSQL to distinguish it by input data types from any other PostgreSQL function of the same name. To keep the example simple, we make the function have the same names at the C level and PostgreSQL level.

  • Finally, note that these operator functions return Boolean values. In practice, all operators defined as index access method strategies must return type boolean, since they must appear at the top level of a WHERE clause to be used with an index. (On the other hand, the support function returns whatever the particular access method expects -- in this case, a signed integer.)

The final routine in the file is the "support routine" mentioned when we discussed the amsupport column of the pg_am table. We will use this later on. For now, ignore it.

Now we are ready to define the operators:

CREATE OPERATOR = (
     leftarg = complex, rightarg = complex,
     procedure = complex_abs_eq,
     restrict = eqsel, join = eqjoinsel
         );

The important things here are the procedure names (which are the C functions defined above) and the restriction and join selectivity functions. You should just use the selectivity functions used in the example (see complex.source). Note that there are different such functions for the less-than, equal, and greater-than cases. These must be supplied or the optimizer will be unable to make effective use of the index.

The next step is to add entries for these operators to the pg_amop relation. To do this, we'll need the OIDs of the operators we just defined. We'll look up the names of all the operators that take two operands of type complex, and pick ours out:

SELECT o.oid AS opoid, o.oprname
    INTO TEMP TABLE complex_ops_tmp
    FROM pg_operator o, pg_type t
    WHERE o.oprleft = t.oid and o.oprright = t.oid
      and t.typname = 'complex';

 opoid  | oprname
--------+---------
 277963 | +
 277970 | <
 277971 | <=
 277972 | =
 277973 | >=
 277974 | >
(6 rows)

(Again, some of your OID numbers will almost certainly be different.) The operators we are interested in are those with OIDs 277970 through 277974. The values you get will probably be different, and you should substitute them for the values below. We will do this with a select statement.

Now we are ready to insert entries into pg_amop for our new operator class. These entries must associate the correct B-tree strategy numbers with each of the operators we need. The command to insert the less-than operator looks like:

INSERT INTO pg_amop (amopclaid, amopstrategy, amopreqcheck, amopopr)
    SELECT opcl.oid, 1, false, c.opoid
        FROM pg_opclass opcl, complex_ops_tmp c
        WHERE
            opcamid = (SELECT oid FROM pg_am WHERE amname = 'btree') AND
            opcname = 'complex_abs_ops' AND
            c.oprname = '<';

Now do this for the other operators substituting for the 1 in the second line above and the < in the last line. Note the order: "less than" is 1, "less than or equal" is 2, "equal" is 3, "greater than or equal" is 4, and "greater than" is 5.

The field amopreqcheck is not discussed here; it should always be false for B-tree operators.

The final step is the registration of the "support routine" previously described in our discussion of pg_am. The OID of this support routine is stored in the pg_amproc table, keyed by the operator class OID and the support routine number.

First, we need to register the function in PostgreSQL (recall that we put the C code that implements this routine in the bottom of the file in which we implemented the operator routines):

CREATE FUNCTION complex_abs_cmp(complex, complex)
    RETURNS integer
    AS 'PGROOT/src/tutorial/complex'
    LANGUAGE C;

SELECT oid, proname FROM pg_proc
    WHERE proname = 'complex_abs_cmp';

  oid   |     proname
--------+-----------------
 277997 | complex_abs_cmp
(1 row)

(Again, your OID number will probably be different.)

We can add the new row as follows:

INSERT INTO pg_amproc (amopclaid, amprocnum, amproc)
    SELECT opcl.oid, 1, p.oid
        FROM pg_opclass opcl, pg_proc p
        WHERE
            opcamid = (SELECT oid FROM pg_am WHERE amname = 'btree') AND
            opcname = 'complex_abs_ops' AND
            p.proname = 'complex_abs_cmp';

And we're done! (Whew.) It should now be possible to create and use B-tree indexes on complex columns.

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