Re: [HACKERS] CLUSTER command progress monitor

On 2019-Sep-06, Michael Paquier wrote:

> Finally comes the question of what do we do for v12?  I am adding in
> CC Peter, Alvaro being already present, who have been involved in the
> commits with CREATE INDEX and REINDEX.  It would be sad to revert a
> this feature, but well I'd rather do that now than regret later
> releasing the feature as it is currently shaped..  Let's see what the
> others think.

As far as I understand, CREATE INDEX is not affected -- only REINDEX is.
Of course, it would be sad to revert even the latter, but it's not as
bleak as reverting the whole thing.

That said, I did spend some time on this type of issue when doing CREATE
INDEX support; you can tell because I defined the columns for block
numbers in a scan separately from CREATE INDEX specific fields,
precisely to avoid multiple commands running concurrently from
clobbering unrelated columns:

/* Block numbers in a generic relation scan */
#define PROGRESS_SCAN_BLOCKS_TOTAL                15
#define PROGRESS_SCAN_BLOCKS_DONE                16

I would say that it's fairly useful to have CLUSTER report progress on
indexes being created underneath, but I understand that it might be too
late to be designing the CLUSTER report to take advantage of the CREATE
INDEX metrics.

I think a workable, not terribly invasive approach is to have REINDEX
process its commands conditionally: have the caller indicate whether
progress is to be reported, and skip the calls if not.  That would
(should) prevent it from clobbering the state set up by CLUSTER.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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