How does the output illustrate implicit elevation of isolation level?
Sunil is technically correct, but it does sound a little confusing, I agree.
The output shows the session is blocked waiting to acquire a U
lock. The definition of the READ COMMITTED
isolation level is that the session will only encounter committed data. SQL Server honours this logical requirement under the default pessimistic (locking) implementation of read committed by holding shared locks just long enough to avoid seeing uncommitted data. These shared locks are normally quickly released (usually just before reading the next row).
Under optimistic (row-versioning) read committed (RCSI
) SQL Server avoids reading uncommitted data by reading the last-committed version of the row at the time the statement started instead.
The sense Sunil is trying to convey is that taking U
locks (instead of brief shared locks or reading versions) represents a (technical) escalation of isolation level (though not to any explicitly named level).
The effective isolation level in this case is not quite REPEATABLE READ
because any U
locks taken (and not converted to X
locks) are released at the end of the statement. This is different from the behaviour of the UPDLOCK
hint, which acquires and holds U
locks (at least) until the end of the transaction. In addition, REPEATABLE READ
generally acquires S
locks (though this is strictly just an implementation detail).
Confusingly, the engine also takes U
locks on the access method when identifying rows to update under default (locking) read-committed. This is a convenience to avoid a common deadlocking scenario without having to specify UPDLOCK
explicitly. I apologise that this is so complicated, but there we are.
How to check for real isolation level "jumpings" in context of some statements?
There is nothing explicitly exposed in query plans to identify cases where the engine temporarily increases the effective isolation level. This might change in a future version of SQL Server. There may be indirect evidence in terms of locks taken, but this is rarely a convenient approach.
When to expect them and why do they occur?
Some of the occasions when internal escalation occurs are (somewhat) documented in Books Online. For example, Understanding Row Versioning-Based Isolation Levels says (among other things worth noting):
In a read-committed transaction using row versioning, the selection of rows to update is done using a blocking scan where an update (U) lock is taken on the data row as data values are read.
The general reason for temporary changes in effective isolation level changes is to avoid data corruption. A list of posts identifying some common cases follows:
Blocking Operators
Large Objects
Lookup with Prefetching
Cascading Referential Integrity
Other common cases (not a complete list):
- Shared locks taken when the query processor verifies foreign key relationships.
- Range locks taken when maintaining an indexed view referencing more than one table.
- Range locks taken when maintaining an index with
IGNORE_DUP_KEY
.
Some of these behaviours may be documented in Books Online, somewhere, but there's no convenient single list that I am aware of.
How can I check for the lock count and ignore all locks which are only N seconds old?
You can't... but I think you're asking the wrong question.
PostgreSQL has many different kinds of locking. The three main ones visible to users are:
- Row locks
- Heavyweight locks
- Advisory locks
A row lock is a lock taken by UPDATE
, DELETE
, SELECT ... FOR UPDATE
, SELECT ... FOR SHARE
, etc, indicating a claim on a row. You cannot see these in the pg_locks
view and they don't consume resources. This is what "a single DML statement can easily obtain millions of locks" refers to.
Heavyweight locks are what people usually mean when they say "lock" without qualifying with the lock type. They're seen in pg_locks
. There are many kinds: locks on relations (tables, views, etc); locks to allow one transaction to wait on another, etc. They're only "heavyweight" compared to PostgreSQL's internal lightweight locking mechanism (LWLocks); you don't have to worry unless you start exceeding max_locks_per_transaction
.
Advisory locks are a utility locking mechanism for applications. They're done with function calls. They're visible in pg_locks
.
Best Answer
Row locks are not permanently stored in the shared lock table, but on the row itself, so you cannot simply query for them.
To figure out which rows in a table are locked by concurrent transactions, you could run