Performance Tuning with SQL Server Dynamic Management Views

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Chapter 4: Transactions
• Shared (S) – shared locks are issued for read-only operations, such as SELECT
statements. They prevent other transactions from updating the same data while the
query is processing. Multiple shared locks can be issued for the same resource, hence
the term "shared."
• Update (U) – update locks prevent data being modified by another transaction in the
time between finding the data that needs to be updated, and actually modifying it.
A U lock is obtained while the data is located, and then this is converted to an
exclusive lock to perform the update. Only a single update lock can be issued
for a given resource, and it must be converted to an exclusive lock before a data
modification can occur.
• Exclusive (X) – exclusive locks are used for data modification requests (INSERT,
UPDATE, and DELETE) to ensure that multiple updates do not occur against the
same resource. Only a single exclusive lock can be placed on a resource at any given
point in time. This prevents any other transaction from reading or modifying the
X-locked data.
• Intent (IX, IU, IS) – intent locks, as their name suggest, indicate an intention to lock
a resource lower in the locking hierarchy. So, for example, immediately before
obtaining an S lock on a row in a table, SQL Server will briefly obtain an Intent
Shared (IS) lock on the table (or page), which will prevent another transaction from
obtaining an exclusive lock on the table. Other types of intent lock are Intent Exclusive
(IX) and Intent Update (IU). All intent locks can be acquired only on pages and tables.
If an X lock is obtained on a key column, then both the page and the table associated
with that key would be assigned an IX lock.
• Conversion – these locks are the result of conversion from one locking mode to
another. There are three types of conversion lock:
• (SIX) – Shared with Intent Exclusive – a transaction currently holds a shared (S)
lock on a resource, then subsequently needs to obtain an exclusive (X) lock on a
component of the resource that is S locked