![]() Using one of our get cursor functions we can now easily count the number of returned cursor records (only 1) by passing the function as argument. Get a reference cursorĬREATE FUNCTION get_refcursor(OUT return_cursor refcursor) We can simplify this by using an OUT argument instead allowing us to omit both the declaration block and return statement, see the Create Function documentation. OPEN return_cursor FOR SELECT 'One Record' We will probably want to use this with cursor producing functions, like: - Get a reference cursor With 50,000 records I had to increase the step size to 1000 before I noticed any improvement over FETCH ALL so unless there is something else worth doing simultaneously the incremental approach is less optimal. GET DIAGNOSTICS _move_count := ROW_COUNT Count the actual number of rows incremented To rectify this we can look up ROW_COUNT and increment by the actual amount moved instead. Increasing the step size does improve performance but the result will be rounded up because FOUND will report success if the cursor has moved at all. Increment the cursor position and count the rows Function returning the number of rows available in the cursorĬREATE FUNCTION get_cursor_size(_cursor refcursor)Īnother approach is to LOOP through the cursor until FOUND returns falsey, however this approach is notably slower than even the FETCH ALL method from the original example in the question. Modify the function to take a refcursor as argument and instead execute the MOVE cursor statement directly which then makes ROW_COUNT available. ![]() The holy grail of PostgreSQL cursor record count methods we've all been waiting for. NOTE: The following examples do not reset the cursor position back to 0 as with the original example, allowing the function to be used with all cursor types especially NO SCROLL cursors which will reject backward movement by raising an error. This is not clearly stipulated in the PostgreSQL documentanion per se, but considering that MOVE does not produce any actual results it might make sense enough to be excused. We require a looping technique to iterate through the stored data. ![]() We utilize several looping and conditional expressions to retrieve data and perform multiple operations. GOTCHA: Using EXECUTE does not update the GET DIAGNOSTICS for MOVE while it does for FETCH, and neither statements will update the FOUND variable. Kamya July 19th, 2022 Are you still stuck finding a basic tutorial on Loop in PostgreSQL Tables in PostgreSQL can be used to store data in various ways. ![]() As it turns out we can indeed retrieve ROW_COUNT diagnostics from a MOVE cursor statement. ![]() Yes it is possible to use MOVE instead of FETCH to count the records in a cursor, with slightly improved performance. Here is a solution proposal, how do you think I can improve it ? (and is it possible to use MOVE instead of FETCH to retrieve the x value ?) - Function returning the number of rows available in the cursorĬREATE FUNCTION get_cursor_size(_cursor_name TEXT)ĮXECUTE format('FETCH FORWARD ALL FROM %I', _cursor_name) ĮXECUTE format('MOVE ABSOLUTE 0 FROM %I', _cursor_name) GET DIAGNOSTICS := ROW_COUNT only works for FETCH but not MOVE. The result is a command tag written to stdout, and I do not know how to retrieve that value in a pgsql function. In that case, a MOVE FORWARD ALL FROM statement returns MOVE x. Since the rows represented by a held cursor are copied into a temporary file or memory area, I am wondering if it is possible to retrieve that number in a straightforward way or if the only solution is to fetch all the records to count them. I would like to retrieve the number of rows that can be fetched by the cursor. But they too will usually have settings where the driver uses cursors behind the scenes without you needing to manually implement it.I have a cursor created using the WITH HOLD option that allows the cursor to be used for subsequent transactions. The usual way to use a cursor in SQL would be in some other programming language with a db driver, like python or JS or Java or Perl. You will have to break out of the loop yourself once it finishes or you get tired of it, like with ctrl-C (on Linux) begin ĭeclare curs cursor for select id from _group order by id asc (If you set log_statement=all, you can see this in action in the log file.) If you really want do it manually for some reason, you could use \watch as an infinite loop. But in psql, you can set FETCH_COUNT, which uses a cursor and FETCH behind the scenes. So there is no way to do it just in SQL or with psql. Neither does psql (in a user-visible way) that I can find. RETURN NEXT kind of looks like it streams via co-routine or something, but it really just accumulates all the rows until the end of the function and returns them at once.Īnd SQL doesn't have any looping constructs. ![]()
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