Re: HyperSQL (HSQLDB) explicit shared (read) lock syntax

Hi,

I assumed there's a construct similar to FOR UPDATE/FOR SHARE that
PostgreSQL has:

http://www.postgresql.org/docs/9.0/static/sql-select.html#SQL-FOR-UPDATE-SHARE

FOR UPDATE - takes an explicit lock on the selected rows FOR SHARE -
takes a shared lock on the selected rows PostgreSQL behaviour is not
relevant here.

The current session obtains the locks that I mentioned. When you use FOR
READ ONLY, you cannot update the rows of the result set (that is, update
via JDBC ResultSet methods).


You can still update the rows of the table by using UPDATE atable SET
... statements in the same session.

Fred



So, related to FOR READ ONLY, I still didn't get this part "you cannot
update the rows of the result set."

The Select ResultSet only retrieves the database rows, but it requires
an UPDATE statement to apply any change. So, the FOR READ ONLY prevents
any update to that rows in this particular transaction, or all
concurring transactions till the end of the current transaction?

Vlad

On Friday, January 30, 2015 3:02 PM, Fred Toussi
<[email protected]> wrote:

The SELECT ... FOR UPDATE directives primarily dictate what you can do
with the result set. If FOR READ ONLY is used, you cannot update the
rows of the result set.

The lock that is created depends on the transaction model. In 2 phase
lock (LOCKS) mode exclusive and shared locks are introduced on the
table. In MVCC mode no lock is introduced at all and any conflict due to
updates by multiple sessions is handled when it occurs.

Fred

On Fri, Jan 30, 2015, at 08:02, Mihalcea Vlad wrote:
> Hi,
>
> According to HSQLDB documentation, there are two select-for-updating
> directives:
>
>
- FOR UPDATE
>
- FOR READ ONLY
>
>
"If the SQL qualifier, FOR UPDATE is used, then all the updatable
columns of the result set become updatable.
>
>
If a cursor is declared with FOR READ ONLY, then it is not updatable.
>
>
In HSQLDB, if FOR READ ONLY or FOR UPDATE is not used then all the
updatable columns of the result set become updatable. This relaxes the
SQL standard rule that in this case limits updatability to only simply
updatable SELECT statements (where all columns are updatable)."
>
>
If the FOR UPDATE takes an exclusive (write) lock, does the FOR READ
ONLY directive take a shared (read) lock instead?
>
>
Vlad Mihalcea

>
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