Asynchronous commit is an option that allows transactions to complete more quickly, at the cost that the most recent transactions may be lost if the database should crash. In many applications this is an acceptable trade-off.
As described in the previous section, transaction commit is normally synchronous: the server waits for the transaction's WAL records to be flushed to permanent storage before returning a success indication to the client. The client is therefore guaranteed that a transaction reported to be committed will be preserved, even in the event of a server crash immediately after. However, for short transactions this delay is a major component of the total transaction time. Selecting asynchronous commit mode means that the server returns success as soon as the transaction is logically completed, before the WAL records it generated have actually made their way to disk. This can provide a significant boost in throughput for small transactions.
Asynchronous commit introduces the risk of data loss. There is a short time window between the report of transaction completion to the client and the time that the transaction is truly committed (that is, it is guaranteed not to be lost if the server crashes). Thus asynchronous commit should not be used if the client will take external actions relying on the assumption that the transaction will be remembered. As an example, a bank would certainly not use asynchronous commit for a transaction recording an ATM's dispensing of cash. But in many scenarios, such as event logging, there is no need for a strong guarantee of this kind.
The risk that is taken by using asynchronous commit is of data loss, not data corruption. If the database should crash, it will recover by replaying WAL up to the last record that was flushed. The database will therefore be restored to a self-consistent state, but any transactions that were not yet flushed to disk will not be reflected in that state. The net effect is therefore loss of the last few transactions. Because the transactions are replayed in commit order, no inconsistency can be introduced — for example, if transaction B made changes relying on the effects of a previous transaction A, it is not possible for A's effects to be lost while B's effects are preserved.
   The user can select the commit mode of each transaction, so that
   it is possible to have both synchronous and asynchronous commit
   transactions running concurrently.  This allows flexible trade-offs
   between performance and certainty of transaction durability.
   The commit mode is controlled by the user-settable parameter
   synchronous_commit, which can be changed in any of
   the ways that a configuration parameter can be set.  The mode used for
   any one transaction depends on the value of
   synchronous_commit when transaction commit begins.
  
   Certain utility commands, for instance DROP TABLE, are
   forced to commit synchronously regardless of the setting of
   synchronous_commit.  This is to ensure consistency
   between the server's file system and the logical state of the database.
   The commands supporting two-phase commit, such as PREPARE
   TRANSACTION, are also always synchronous.
  
   If the database crashes during the risk window between an
   asynchronous commit and the writing of the transaction's
   WAL records,
   then changes made during that transaction will be lost.
   The duration of the
   risk window is limited because a background process (the “WAL
   writer”) flushes unwritten WAL records to disk
   every wal_writer_delay milliseconds.
   The actual maximum duration of the risk window is three times
   wal_writer_delay because the WAL writer is
   designed to favor writing whole pages at a time during busy periods.
  
An immediate-mode shutdown is equivalent to a server crash, and will therefore cause loss of any unflushed asynchronous commits.
   Asynchronous commit provides behavior different from setting
   fsync = off.
   fsync is a server-wide
   setting that will alter the behavior of all transactions.  It disables
   all logic within PostgreSQL that attempts to synchronize
   writes to different portions of the database, and therefore a system
   crash (that is, a hardware or operating system crash, not a failure of
   PostgreSQL itself) could result in arbitrarily bad
   corruption of the database state.  In many scenarios, asynchronous
   commit provides most of the performance improvement that could be
   obtained by turning off fsync, but without the risk
   of data corruption.
  
   commit_delay also sounds very similar to
   asynchronous commit, but it is actually a synchronous commit method
   (in fact, commit_delay is ignored during an
   asynchronous commit).  commit_delay causes a delay
   just before a transaction flushes WAL to disk, in
   the hope that a single flush executed by one such transaction can also
   serve other transactions committing at about the same time.  The
   setting can be thought of as a way of increasing the time window in
   which transactions can join a group about to participate in a single
   flush, to amortize the cost of the flush among multiple transactions.