Unusual problem. A sequence is incrementing by 2 when increment_by = 1, think it's caused by an INSERT rule

Hi all,

I'm wondering if anyone has any ideas why an insert rule on a table with
a sequence would cause the sequence to be incremented by two on every
insert.

The table definition is :

CREATE TABLE "z_codes" (
"idnum" integer DEFAULT nextval('z_codes_idnum_seq'::text) NOT
NULL,
"description" character varying(30) NOT NULL,
"enabled" boolean DEFAULT 't'::bool NOT NULL,
CONSTRAINT "z_codes_description" CHECK ((length(description) <
31)),
Constraint "z_codes_pkey" Primary Key ("idnum")
);

The RULE is :

CREATE RULE protect_z_insert AS ON INSERT TO z_codes WHERE (new.idnum =
0) DO INSTEAD NOTHING;

The present value of the sequence is :

foo=# select * from z_codes_idnum_seq;
sequence_name | last_value | increment_by | max_value |
min_value | cache_value | log_cnt | is_cycled | is_called
-------------------+------------+--------------+------------+-----------+-------------+---------+-----------+-----------
z_codes_idnum_seq | 98 | 1 | 2147483647 | 1
| 1 | 23 | f | t
(1 row)

When doing an insert that uses the default value
(nextval('z_codes_idnum_seq')) for the idnum field, the inserted value
is only ever incremented by 2, never by 1. This is confusing, as I
don't see how the rule is even applicable as the new.idnum is not going
to be 0 in any of these cases.

Any thoughts?

Regards and best wishes,

Justin Clift

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"My grandfather once told me that there are two kinds of people: those
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