Message-ID: <cb623e32-4cba-4900-b379-dcc7e6da7023@cloud.gatewaynet.com>
Date: Mon, 11 Nov 2024 10:27:01 +0200
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Subject: Re: Duplicate key error
To: pgsql-general@lists.postgresql.org
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From: Achilleas Mantzios - cloud <a.mantzios@cloud.gatewaynet.com>
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On 11/11/24 07:40, yudhi s wrote:

> We have a merge query as below for a partition table which is range 
> partitioned on a truncated date column 'part_date'. And the only 
> unique key in this table is a composite primary key on (id, 
> part_date). And this merge queries ON condition is based on one of the 
> columns i.e ID which is the leading column of the PK.So it means it 
> will ensure no duplication happens for ID values, but still we are 
> seeing "duplicate key" error. So what is the possible reason here or 
> are we encountering any buggy behaviour here?
>
> WITH source_data (col1, col2, col3.....col29) AS (VALUES ($1, 
> $2::date, $3::timestamptz, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, 
> $14, $15, $16, $17, $18, $19, $20, $21, $22, $23, $24, $25, $26, 
> $27::timestamptz, $28, $29::timestamptz)) MERGE INTO cpod.TAB1 AS 
> target USING source_data AS source ON target.ID = source.ID WHEN 
> MATCHED THEN UPDATE SET ....) WHEN NOT MATCHED THEN INSERT (...) 
> VALUES (....);
>
> Error:
> ERROR: duplicate key value violates unique constraint "TAB1_pkey"
> Detail: Key (ID, part_date)=(XXXXXXXXX, 2024-11-04) already exists.

You have to use the whole composite unique key (including part_date) 
when matching against source_data .

If you had uniqueness on "ID" and then added "part_name" to the key, 
you'd still had uniqueness.

BUT, reversely, if you have uniqueness on the pair (ID, part_date) there 
is no guaranteed uniqueness on ID alone, hence your ERROR.

>
> Regards
> Yudhi
>