Re: " It appears that the second row was in place originally, then got updated
by a trigger (and even deleted later on, although it doesn't appear that the
delete transaction got committed), and then the first row was inserted within
the same transaction that updated the second row."
If you have BEFORE triggers, and a BEFORE trigger signaled failure with RETURN
NULL, then this is one known (and documented) issue that I think could cause
the behavior you're reporting:
https://www.postgresql-archive.org/BEFORE-triggers-that-return-NULL-can-circumvent-referential-integrity-tt6056390.html#none
It's hard to say if this is the cause or not, but if you have any BEFORE
triggers that RETURN NULL, you might want to review the documentation very
carefully.
thanks,
/Jim F
On 12/5/19, 6:45 PM, "Tomas Vondra" <tomas.von...@2ndquadrant.com> wrote:
On Thu, Dec 05, 2019 at 09:14:12PM +0000, Alex Adriaanse wrote:
>We have a Postgres 10 database that we recently upgraded to Postgres 12
using pg_upgrade. We recently discovered that there are rows in one of the
tables that have duplicate primary keys:
>
>record_loader=# \d loader.sync
> Table "loader.sync"
> Column | Type | Collation | Nullable |
Default
>-------------------+--------------------------+-----------+----------+---------
> source | text | | not null |
> natural_key | text | | not null |
> payload | jsonb | | |
> dispatched | timestamp with time zone | | not null |
now()
> initial_load_id | text | | |
> deleted_load_id | text | | |
> created_timestamp | timestamp with time zone | | |
now()
> updated_timestamp | timestamp with time zone | | |
now()
> deleted_timestamp | timestamp with time zone | | |
>Indexes:
> "sync_pkey" PRIMARY KEY, btree (source, natural_key)
>Publications:
> "debezium"
>
>This table is modified via triggers that fire off when a COPY command
inserts many rows into another table.
>
>Here are two example duplicate rows:
>
># SELECT xmin, xmax, cmin, cmax, source, md5(natural_key) AS
natural_key_hash, dispatched, created_timestamp, updated_timestamp,
deleted_timestamp FROM loader.sync WHERE (source, natural_key) = ('ok_lease',
'...') ORDER BY xmin::text::int, cmin::text::int;
>-[ RECORD 1 ]-----+---------------------------------
>xmin | 116649
>xmax | 0
>cmin | 5304404
>cmax | 5304404
>source | ok_lease
>natural_key_hash | de3e9a567b90025c3399c4c63c823fe9
>dispatched | 2019-11-24 05:09:36.099686+00
>created_timestamp | 2019-11-24 05:09:36.099686+00
>updated_timestamp | 2019-11-24 05:09:36.099686+00
>deleted_timestamp |
>-[ RECORD 2 ]-----+---------------------------------
>xmin | 116649
>xmax | 118583
>cmin | 5312208
>cmax | 5312208
>source | ok_lease
>natural_key_hash | de3e9a567b90025c3399c4c63c823fe9
>dispatched | 2019-11-10 05:09:24.214964+00
>created_timestamp | 2019-05-17 21:24:19.558219+00
>updated_timestamp | 2019-11-24 05:09:36.099686+00
>deleted_timestamp | 2019-11-24 05:09:36.099686+00
>
>It appears that the second row was in place originally, then got updated
by a trigger (and even deleted later on, although it doesn't appear that the
delete transaction got committed), and then the first row was inserted within
the same transaction that updated the second row.
>
>Another example:
>-[ RECORD 1 ]-----+---------------------------------
>xmin | 116649
>xmax | 0
>cmin | 5304403
>cmax | 5304403
>source | ok_lease
>natural_key_hash | 1c8031348701a32cb5fee26839d6b0b4
>dispatched | 2019-11-10 05:09:24.214964+00
>created_timestamp | 2019-05-31 06:00:33.765547+00
>updated_timestamp | 2019-11-24 05:09:36.099686+00
>deleted_timestamp | 2019-11-24 05:09:36.099686+00
>-[ RECORD 2 ]-----+---------------------------------
>xmin | 116649
>xmax | 0
>cmin | 5304404
>cmax | 5304404
>source | ok_lease
>natural_key_hash | 1c8031348701a32cb5fee26839d6b0b4
>dispatched | 2019-11-24 05:09:36.099686+00
>created_timestamp | 2019-11-24 05:09:36.099686+00
>updated_timestamp | 2019-11-24 05:09:36.099686+00
>deleted_timestamp |
>
>Both examples have in common that the two duplicate rows were touched
within the same transaction.
>
>This database runs inside Docker, with the data directory bind-mounted to
a reflink-enabled XFS filesystem. The VM is running Debian's 4.19.16-1~bpo9+1
kernel inside an AWS EC2 instance. We have Debezium stream data from this
database via pgoutput.
>
>Recreating the primary key confirms that the constraint doesn't (or at
least shouldn't) permit these duplicate rows:
>
>record_loader=# BEGIN;
>BEGIN
>record_loader=# ALTER TABLE loader.sync DROP CONSTRAINT sync_pkey;
>ALTER TABLE
>record_loader=# ALTER TABLE loader.sync ADD CONSTRAINT sync_pkey PRIMARY
KEY (source, natural_key);
>ERROR: could not create unique index "sync_pkey"
>DETAIL: Key (source, natural_key)=(ok_lease, ...) is duplicated.
>CONTEXT: parallel worker
>
>Any ideas on what might cause this behavior?
>
Not sure. At first I thought maybe this might be due to collations
changing and breaking the index silently. What collation are you using?
A couple questions:
1) When you do the queries, do they use index scan or sequential scan?
Perhaps it does sequential scan, and if you force index scan (e.g. by
rewriting the query) it'll only find one of those rows.
2) Can you check in backups if this data corruption was present in the
PG10 cluster, before running pg_upgrade?
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
