How can a table violate it's own primary key index?

Question

I have a PostgreSQL database which has a table with the primary key applied to three columns. According to the database, there is an index on the key:

Indexes:
    "full_log_pkey" PRIMARY KEY, btree (server_name, line_number, log_generation)

Yet some simple tests reveal that I have duplicate keys:

select count(*) from full_log;
  count
----------
 60644405

select count(*) from 
    (select distinct server_name, 
                     line_number, 
                     log_generation 
            from     full_log) as foo;
  count
----------
 60636564

Clearly there are fewer distinct rows (based on the primary key) than there are rows. My question is, how is this possible?

Edit: The full table definition is such:

                 Table "public.full_log"
     Column     |            Type             | Modifiers
----------------+-----------------------------+-----------
 activity       | character(1)                |
 archivaldate   | timestamp without time zone |
 media_type     | character varying(5)        |
 vsn            | text                        |
 archive_set    | character varying(20)       |
 copy           | smallint                    |
 file_start     | integer                     |
 file_offset    | integer                     |
 fs_name        | character varying(20)       |
 inode          | double precision            |
 file_length    | bigint                      |
 file_type      | character(1)                |
 overflow       | integer                     |
 device_number  | integer                     |
 server_name    | text                        | not null
 path           | text                        |
 line_number    | integer                     | not null
 log_generation | integer                     | not null
Indexes:
    "full_log_pkey" PRIMARY KEY, btree (server_name, line_number, log_generation)
Foreign-key constraints:
    "full_log_server_name_fkey" FOREIGN KEY (server_name) REFERENCES servers(server_name)
Rules:
    insert_update_full_log AS
    ON INSERT TO full_log
   WHERE (EXISTS ( SELECT full_log.activity, full_log.archivaldate, full_log.media_type, full_log.vsn, full_log.archive_set, full_log.copy, full_log.file_start, full_log.file_offset, full_log.fs_name, full_log.inode, full_log.file_length, full_log.file_type, full_log.overflow, full_log.device_number, full_log.server_name, full_log.path, full_log.line_number, full_log.log_generation
           FROM full_log
          WHERE full_log.server_name = new.server_name AND full_log.line_number = new.line_number AND full_log.log_generation = new.log_generation)) DO INSTEAD  UPDATE full_log SET activity = new.activity, archivaldate = new.archivaldate, media_type = new.media_type, vsn = new.vsn, archive_set = new.archive_set, copy = new.copy, file_start = new.file_start, file_offset = new.file_offset, fs_name = new.fs_name, inode = new.inode, file_length = new.file_length, file_type = new.file_type, overflow = new.overflow, device_number = new.device_number, path = new.path
  WHERE full_log.server_name = new.server_name AND full_log.line_number = new.line_number AND full_log.log_generation = new.log_generation

For an example of duplicate rows:

 select * from full_log where line_number = 6332986;
 activity |    archivaldate     | media_type |  vsn   | archive_set | copy | file_start | file_offset | fs_name |   inode    | file_length | file_type | overflow | device_number | server_name |                                           path                                            | line_number | log_generation
----------+---------------------+------------+--------+-------------+------+------------+-------------+---------+------------+-------------+-----------+----------+---------------+-------------+-------------------------------------------------------------------------------------------+-------------+----------------
 A        | 2010-10-13 10:49:49 | ti         | Z00711 | lcbp_rel    |    1 |     226237 |      779099 | lcbp    | 21798068.3 |    31198108 | f         |        0 |          8511 | redact      | wdl/delivery/irishparis_2010_09/MSE2_Histoire des rois d'Angleterre/MSE2_239.TIF          |     6332986 |              1
 A        | 2010-10-13 10:49:49 | ti         | Z00711 | lcbp_rel    |    1 |     226237 |      779099 | lcbp    | 21798068.3 |    31198108 | f         |        0 |          8511 | redact      | wdl/delivery/irishparis_2010_09/MSE2_Histoire des rois d'Angleterre/MSE2_239.TIF          |     6332986 |              1
(2 rows)

Answer 1

What does this query return?

select server_name, line_number, log_generation 
from full_log
group by server_name, line_number, log_generation
having count(*) > 1

It might help to compare that to

select line_number, log_generation 
from full_log
group by line_number, log_generation
having count(*) > 1

but it might not. I think this clause

WHERE (EXISTS ( SELECT full_log.activity, 
                       full_log.archivaldate, 
                       full_log.media_type, 
                       full_log.vsn, 
                       full_log.archive_set, 
                       full_log.copy, 
                       full_log.file_start, 
                       full_log.file_offset, 
                       full_log.fs_name, 
                       full_log.inode, 
                       full_log.file_length, 
                       full_log.file_type, 
                       full_log.overflow, 
                       full_log.device_number, 
                       full_log.server_name, 
                       full_log.path, 
                       full_log.line_number, 
                       full_log.log_generation
               FROM full_log
               WHERE full_log.server_name = new.server_name 
                 AND full_log.line_number = new.line_number 
                 AND full_log.log_generation = new.log_generation)) 

can be simplified to this clause. (Although I don't think this contributes to the problem.)

WHERE (EXISTS ( SELECT full_log.server_name, 
                       full_log.line_number, 
                       full_log.log_generation
                FROM full_log
                WHERE full_log.server_name = new.server_name 
                  AND full_log.line_number = new.line_number 
                  AND full_log.log_generation = new.log_generation)) 

You said PostgreSQL drops and recreates the index when when you alter the data type of a non-key column. I don't see that happening here, and I'm not sure I've ever seen that happen. I might not have noticed if the change succeeded, and I don't routinely change the data type of a column. (Now that I've said that, I couldn't begin to tell you the last time I did that.) I now have PostgreSQL 9.0.2 here.

Answer 2

Are you using table inheritance? If so, the PK is not enforced across children (at least, not in 8.2).