Question

I have a complex select query and a huge table.

I’m running this select statement, meanwhile an Update statement arrives and tries to update the table.

IMHO – update requires an exclusive lock – so the update statement will have to wait till the select command is finished.

1. Am I right ?

2. what can I do in order to: execute the complex select, and also let the update command run (currently I don’t care about dirty data)

Answer 1

Yes – to a degree.

How long a SELECT holds on to a shared lock is depending on the isolation level of the transaction:

• READ UNCOMMITTED – no shared lock is acquired at all – UPDATE is not blocked
• READ COMMITTED – shared lock is acquired just for the duration of reading the data – UPDATE might be blocked for a very short period of time
• REPEATABLE READ and SERIALIZABLE – shared lock is acquired and held on to until the end of the transaction – UPDATE is blocked until the SELECT transaction ends

Technically, the UPDATE statement first gets an UPDATE lock – which is compatible with a shared lock (as used by the SELECT) – for the duration of the time while it’s reading the current values of the rows to be updated.

Once that’s done, the Update lock is escalated to an exclusive lock for the new data to be written to the table.