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  1. Doc: minor improvements for plpgsql "Transaction Management" section.

  1. BUG #18531: err when using 'current of' with incremental COMMIT

    The Post Office <noreply@postgresql.org> — 2024-07-09T11:00:29Z

    The following bug has been logged on the website:

Bug reference:      18531
Logged by:          Dzmitry Jachnik
Email address:      dzja112@gmail.com
PostgreSQL version: 14.2
Operating system:   x86_64 GNU/Linux
Description:        

I tried using 'current of' syntax (like doc. 43.7.3.3.)
and had have ERROR: cursor "bulk_collection_cur" is held from a previous
transaction

If using regular ` WHERE id = l_rec.id; ` I hadn't any error

EXAMPLE:
```

CREATE OR REPLACE PROCEDURE crm_import.etlp_deals(IN i_id bigint DEFAULT
NULL::bigint, IN i_limit integer DEFAULT null)
 LANGUAGE plpgsql
AS $procedure$ 
DECLARE

  result jsonb;
  COUNTER int := 0;
 
  bulk_collection_cur CURSOR FOR
  		SELECT *
          FROM crm_import.deals dew
         WHERE COALESCE(dew.etl_stage,'NOT_LOADED') not IN ('LOADED',
'IGNORE')
--           AND dew.id = any(l_id)
           AND (dew.id = i_id OR i_id IS null)
         ORDER BY dew.load_date, dew.dt
           FOR UPDATE ;
begin

	<<COLLECTION>>
	BEGIN 
		FOR l_rec IN bulk_collection_cur
		LOOP 
	    	COUNTER := COUNTER + 1;
			RESULT := crm_import.etl_deals(row_to_json(l_rec.*)::jsonb);
		     
	    	UPDATE crm_import.deals dsp
	           SET etl_protocol = RESULT
	        -------------------
	        --v.1 SQL Error [24000]: ERROR: cursor "bulk_collection_cur" is
held from a previous transaction
	    	WHERE CURRENT OF bulk_collection_cur;
	        --v.2 WITHOUT ERROR
	        --WHERE id = l_rec.id; 					
			-------------------
	    	RAISE NOTICE 'counter= %', counter;
			IF mod(counter, i_limit) = 0 THEN
				RAISE NOTICE 'COMMIT';
		    	COMMIT;
		    END IF;
		END LOOP;
	END COLLECTION;

end $procedure$
;
```
That combine 'current of' and 'commit' at one loop is bad idea?

  2. Re: BUG #18531: err when using 'current of' with incremental COMMIT

    Tom Lane <tgl@sss.pgh.pa.us> — 2024-07-15T16:07:40Z

    PG Bug reporting form <noreply@postgresql.org> writes:
> That combine 'current of' and 'commit' at one loop is bad idea?

Yeah, that's not going to work.  The first COMMIT auto-holds the
cursor [1], and after that there is not an active cursor query for
the WHERE CURRENT OF to refer to.  Even if that data were somehow
retained, the COMMIT would have dropped the tuple locks taken by
the cursor query, so that the semantics would be a lot squishier
than you'd want.

The discussion at [1] fails to mention that there are any such
gotchas implicit in the auto-hold behavior, so I added a little
bit of text [2].

			regards, tom lane

[1] https://www.postgresql.org/docs/current/plpgsql-transactions.html
[2] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=a0899c0a97d19b0c330ff885af4b78b3e7efb100