postgresql - How to manually lock and unlock a row? -

i trying circumvent double-writes locking , unlocking rows.

i have table personnel , user tries edit row, want lock row.

so

begin; // guess have lock somehow here select * personnel id = 12; commit; 

and once edit been made, want submit update in same style:

begin; //unlocking update personnel set ... id = 12; commit; 

in time between, when user tries edit same row, message.

locks released @ end of transaction. need keep transaction open uphold lock , in single transaction:

begin; select * personnel id = 12 for update nowait;  -- row level locking commit;  begin; update personnel set ... id = 12; commit;

alternatively can use less restrictive for share clause.
details in manual on select in chapter "the locking clause".

concurrent reads allowed. quoting chapter "row-level locks" in manual:

in addition table-level locks, there row-level locks, can exclusive or shared locks. exclusive row-level lock on specific row automatically acquired when row updated or deleted. lock held until transaction commits or rolls back, table-level locks. row-level locks not affect data querying; block writers same row.

to have concurrent attempts same fail immediately, use nowait option - all competing queries.