Re: ALTER tbl rewrite loses CLUSTER ON index

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Michael Paquier <michael(at)paquier(dot)xyz>
Cc: Justin Pryzby <pryzby(at)telsasoft(dot)com>, Amit Langote <amitlangote09(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: ALTER tbl rewrite loses CLUSTER ON index
Date: 2020-04-02 07:38:36
Message-ID: 20200402073836.GA17269@alvherre.pgsql
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On 2020-Apr-02, Michael Paquier wrote:

> Now, regarding patch 0002, note that you have a problem for this part:
> - tuple = SearchSysCache1(INDEXRELID, ObjectIdGetDatum(indexOid));
> - if (!HeapTupleIsValid(tuple)) /* probably can't happen */
> - {
> - relation_close(OldHeap, AccessExclusiveLock);
> - pgstat_progress_end_command();
> - return;
> - }
> - indexForm = (Form_pg_index) GETSTRUCT(tuple);
> - if (!indexForm->indisclustered)
> + if (!get_index_isclustered(indexOid))
> {
> - ReleaseSysCache(tuple);
> relation_close(OldHeap, AccessExclusiveLock);
> pgstat_progress_end_command();
> return;
> }
> - ReleaseSysCache(tuple);
> On an invalid tuple for pg_index, the new code would issue an error,
> while the old code would just return.

I don't think we need to worry about that problem, because we already
checked that the pg_class tuple for the index is there two lines above.
The pg_index tuple cannot have gone away on its own; and the index can't
be deleted either, because cluster_rel holds AEL on the table. There
isn't "probably" about the can't-happen condition, is there?

Álvaro Herrera
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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