pq: unexpected Parse response 'D'

[rothskeller]

You can probably close this issue immediately; it was user error on my part. But I searched your issues list looking for help debugging it and couldn't find any, so I'm adding this issue for future people searching for it.

In my case, this wonderfully cryptic message resulted from trying to start a new statement before reading all of the rows of the preceding statement. Should the future searcher get this error, that would be the first thing to look for.

[maddyblue]

This is a bug. Which commit were you running? We just added something similar to this in the most recent commit (dd1fe20).

[rothskeller]

I'm running 8837942.

[rothskeller]

What I was trying to do, by the way, was read the rows of a Query and, while processing each one, insert a corresponding row to a different table; all of this within the same transaction. Of course I should have realized that's not legal. It's too bad, though; I really don't want to cache all of the data in memory, because there's a lot of it. But I can't think of a better answer.

[tamird]

@rothskeller you can probably do what you're after using cursors: https://www.postgresql.org/docs/current/static/plpgsql-cursors.html

[rothskeller]

Bless you, @tamird, that's exactly what I need. Learn something new every day....

[hypnoglow]

Example on using cursors for your case (with some random data):

if _, err := tx.Exec(`
	DECLARE orders_cursor CURSOR FOR
	SELECT id
	FROM order
	WHERE client_id = $1
	ORDER BY createdAt
	FOR UPDATE
	`,
	client.ID,
); err != nil {
	return err
}
defer tx.Exec("CLOSE orders_cursor")

var id int64

for {
	if err := tx.QueryRow(
		`FETCH NEXT FROM orders_cursor`,
	).Scan(&id); err != nil {
		if err == sql.ErrNoRows {
			// End of rows.
			break
		}

		return err
	}

	if _, err := tx.Exec(`
		UPDATE order
		SET amount = amount - 1
		WHERE CURRENT OF orders_cursor
		`,
	); err != nil {
		return err
	}
}

return nil

[timbunce]

Thanks for the example code @hypnoglow.

What I was trying to do, by the way, was read the rows of a Query and, while processing each one, insert a corresponding row to a different table; all of this within the same transaction. Of course I should have realized that's not legal.

It works in the official libpq (and so all drivers that build on it or emulate it closely) because libpq defaults to fetching and buffering all the rows in memory. It's arguably a cheat but a very effectively one that's very widely used.

See also #81.