Date: Sun, 21 Mar 2021 15:06:45 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Justin Pryzby <pryzby@telsasoft.com>
Cc: Amit Langote <amitlangote09@gmail.com>,
	pgsql-hackers@lists.postgresql.org
Subject: Re: ALTER TABLE .. DETACH PARTITION CONCURRENTLY
Message-ID: <20210321180645.GA2437@alvherre.pgsql>
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On 2021-Mar-19, Alvaro Herrera wrote:

> diff --git a/src/backend/utils/cache/partcache.c b/src/backend/utils/cache/partcache.c
> index 0fe4f55b04..6dfa3fb4a8 100644
> --- a/src/backend/utils/cache/partcache.c
> +++ b/src/backend/utils/cache/partcache.c
> @@ -352,16 +352,9 @@ generate_partition_qual(Relation rel)
>  		return copyObject(rel->rd_partcheck);
>  
>  	/*
> -	 * Obtain parent relid; if it's invalid, then the partition is being
> -	 * detached.  The constraint is NIL in that case, and let's cache that.
> +	 * Obtain parent relid.  XXX explain why we need this
>  	 */
> -	parentrelid = get_partition_parent(RelationGetRelid(rel));
> -	if (parentrelid == InvalidOid)
> -	{
> -		rel->rd_partcheckvalid = true;
> -		rel->rd_partcheck = NIL;
> -		return NIL;
> -	}
> +	parentrelid = get_partition_parent(RelationGetRelid(rel), true);

One thing that makes me uneasy about this, is that I don't understand
how does this happen with your test of two psqls doing attach/detach.
(It is necessary for the case when the waiting concurrent detach is
canceled, and so this fix is necessary anyway).  In your test, no
waiting transaction is ever cancelled; so what is the period during
which the relation is not locked that causes this code to be hit?  I
fear that there's a bug in the lock protocol somewhere.

-- 
Álvaro Herrera       Valdivia, Chile
"In Europe they call me Niklaus Wirth; in the US they call me Nickel's worth.
 That's because in Europe they call me by name, and in the US by value!"