When shared locks are relesed in read committed.

Question

hi,
https://msdn.microsoft.com/en-us/library/cc546518.aspx
following lines are form above link
"shared locks are acquired for read operations, but they are released as soon as they have been granted"
Q1) Are they released after the stmt is executed " select * from a" or they are realease after the row is read and it continues with second row.
Q2) As far as acquiring of shared lock is concerned i think they are acquired row by row , and mean  while other transaction can update rows which are not having shared lock till now , like last rows?
yours sincerley

Answer 1

That means.
if a stmt select three rows.
Q1)Then will it get shared lock for first row, reads it, release the lock, then take shared lock on second row and read it then release it again after reading the second row , then it will get thired row lock ....?
Q2) As far as acquiring of shared lock is concerned i think they are acquired row by row , and mean  while
other transaction can update rows which are not having shared lock till now , like last rows?
Q1) Yes, in read committed mode when your query does not have any hints to change the behavior, if a query is only reading data from a table (like a SELECT statement), then the lock on each row is taken just before the row is read, the row is read, the lock
is freed and it goes on to the next row and does the same process on that row (lock, read, free).
Q2) Yes, the locks are acquired on a row basis.  Since they are then freed, your query will not stop other connections from updating rows in the table(s) you are reading except for the vary short period of time the lock is held on each row.  So
your query could read lock row 1, read row 1, free the lock on row1, get a lock on row 2 and be in the process of reading row 2 when some other connection updates row 1.  That connection would be allowed to update row 1.
Tom