Postgresql函数，它锁定表，更新值并返回行

Question

For my Hi/Lo implementation, I need a function which will acquire exclusive table lock, update value and select one row. I came up with the following code:

CREATE OR REPLACE FUNCTION bb_next_hi(tbl varchar(35))  
RETURNS setof record AS
$$ 
    LOCK TABLE hilo IN ACCESS EXCLUSIVE MODE;
    OPEN ref FOR SELECT hi as "Hi", lo as "Lo", "table" as "Table" FROM hilo WHERE "table" = $1;
    UPDATE hilo SET hi = hi + 1 WHERE "table" = $1
    RETURN ref;

$$ LANGUAGE plpgsql;

However, when invoking the function, it returns not a row, but one column with a content similar to "unnamed portal 3". I guess I should iterate over returned ref (but how)?

Another approach that I could use is to use UPDATE RETURNING statement, but I'm not sure if race conditions can occur in this case. Any help would be appreciated. Thanks

Answer 1

This seems far more simple to me:

CREATE OR REPLACE FUNCTION bb_next_hi(tbl varchar(35))  
RETURNS setof record AS
$$ 
BEGIN
    LOCK TABLE hilo IN ACCESS EXCLUSIVE MODE;
    RETURN QUERY UPDATE hilo SET hi = hi + 1 WHERE "table" = $1
        RETURNING hi as "Hi", lo as "Lo", "table" as "Table";
END;
$$ LANGUAGE plpgsql;

I also usually define RETURNS TABLE (col1 varchar, col2 varchar...) instead of RETURNS setof record .

Answer 2

You are using a cursor variable (which you did not declare, and missing some other PL/pgSQL required elements too) and you return that. Cursor references are indeed called "portals" so that explains your output. A better approach is this:

CREATE OR REPLACE FUNCTION bb_next_hi(tbl varchar(35))  
RETURNS TABLE ("Hi" int, "Lo" int, "Table" varchar(35)) AS $$
BEGIN
    LOCK TABLE hilo IN ACCESS EXCLUSIVE MODE;
    RETURN QUERY SELECT hi, lo, $1 FROM hilo WHERE "table" = $1;
    UPDATE hilo SET hi = hi + 1 WHERE "table" = $1;
    RETURN;
END;
$$ LANGUAGE plpgsql STRICT;

But do you really need an exclusive lock on the table? There are much less invasive approaches to getting a consistent table snapshot.