背景
PostgreSQL 9.6动态视图pg_stat_activity新增了wait_event_type, wait_event的等待事件展示。
当会话处于等待状态时，wait_event与wait_event_type非空，表示会话正在等待的类型。
根据等待信息，可以了解当前会话的状态。
将来也可以通过插件的形式，掌握数据库在某个时间段内的等待事件统计，更好的诊断数据库的健康状态。
已有的插件如下
https://github.com/postgrespro/pg_wait_sampling
用于对等待事件进行采样。
例子
pg_wait_sampling_history view – history of wait events obtained by sampling into
in-memory ring buffer.
Column name Column type Description
pid int4 Id of process
ts timestamptz Sample timestamp
event_type text Name of wait event type
event text Name of wait event
pg_wait_sampling_profile view – profile of wait events obtained by sampling into
in-memory hash table.
Column name Column type Description
pid int4 Id of process
event_type text Name of wait event type
event text Name of wait event
count text Count of samples
pg_wait_sampling_reset_profile() function resets the profile.
The work of wait event statistics collector worker is controlled by following
GUCs.
Parameter name Data type Description Default value
pg_wait_sampling.history_size int4 Size of history in-memory ring buffer 5000
pg_wait_sampling.history_period int4 Period for history sampling in milliseconds 10
pg_wait_sampling.profile_period int4 Period for profile sampling in milliseconds 10
pg_wait_sampling.profile_pid bool Whether profile should be per pid true
PostgreSQL 9.6 等待事件
详见
https://www.postgresql.org/docs/9.6/static/monitoring-stats.html
pg_stat_activity 视图新增等待事件列，可以观察到会话当前的等待。
1. wait_event_type
表示等待时间的类别，如果backend处于等待状态则有内容，否则为空。
类别如下
1.1 LWLockNamed:
命名的轻量锁，这种锁的目的是用于保护内存中的数据结构，防止并发的问题。
The backend is waiting for a specific named lightweight lock.
Each such lock protects a particular data structure in shared memory.
1.2 LWLockTranche:
分组轻量锁，没有细分名字，只是笼统的分类。
The backend is waiting for one of a group of related lightweight locks.
All locks in the group perform a similar function;
1.3 Lock:
重量级锁，用于保护SQL可见对象，例如表。 也可以用于保护存储，例如扩展表时。
见 src/include/storage/lock.h
The backend is waiting for a heavyweight lock.
Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables.
However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension.
1.4 BufferPin:
bufferpin用于保护数据库data buffer中的数据，例如保护游标访问的数据。
The server process is waiting to access to a data buffer during a period when no other process can be examining that buffer.
Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question.
2. wait_event
表示wait_event_type中对应的详细的等待事件。
如果当前backend处于等待状态，则有值，否则为空
Wait event name if backend is currently waiting, otherwise NULL.
2.1 LWLockNamed
ShmemIndexLock
Waiting to find or allocate space in shared memory.
OidGenLock
Waiting to allocate or assign an OID.
XidGenLock
通常出现在高并发的请求事务号时
Waiting to allocate or assign a transaction id.
ProcArrayLock
通常出现在高并发的请求事务号，并且开启了old_snapshot_threshold时
Waiting to get a snapshot or clearing a transaction id at transaction end.
SInvalReadLock
Waiting to retrieve or remove messages from shared invalidation queue.
SInvalWriteLock
Waiting to add a message in shared invalidation queue.
WALBufMappingLock
Waiting to replace a page in WAL buffers.
WALWriteLock
wal刷盘较慢时，可以提高wal writer频率，或者加大BUFFER，或者提高目标盘的IOPS，降低目标盘的RT。
Waiting for WAL buffers to be written to disk.
ControlFileLock
如果产生XLOG确实很频繁，并且没有办法降低，可以使用较大的XLOG文件，最大64MB。
通常这个很少见。
Waiting to read or update the control file or creation of a new WAL file.
CheckpointLock
Waiting to perform checkpoint.
CLogControlLock
Waiting to read or update transaction status.
SubtransControlLock
Waiting to read or update sub-transaction information.
MultiXactGenLock
Waiting to read or update shared multixact state.
MultiXactOffsetControlLock
Waiting to read or update multixact offset mappings.
MultiXactMemberControlLock
Waiting to read or update multixact member mappings.
RelCacheInitLock
Waiting to read or write relation cache initialization file.
CheckpointerCommLock
检查点分三步（write, sync_file_range, fsync），表示fsync请求出现等待，需要提高IO，或者减少fsync时的dirty page。
Waiting to manage fsync requests.
TwoPhaseStateLock
Waiting to read or update the state of prepared transactions.
TablespaceCreateLock
Waiting to create or drop the tablespace.
BtreeVacuumLock
频繁出现，说明索引字段被频繁更新。
Waiting to read or update vacuum-related information for a B-tree index.
AddinShmemInitLock
Waiting to manage space allocation in shared memory.
AutovacuumLock
Autovacuum worker or launcher waiting to update or read the current state of autovacuum workers.
AutovacuumScheduleLock
说明autovacuum单表比较慢，看看是否可以关闭autovacuum的SLEEP调度。
Waiting to ensure that the table it has selected for a vacuum still needs vacuuming.
SyncScanLock
Waiting to get the start location of a scan on a table for synchronized scans.
RelationMappingLock
Waiting to update the relation map file used to store catalog to filenode mapping.
AsyncCtlLock
Waiting to read or update shared notification state.
AsyncQueueLock
Waiting to read or update notification messages.
SerializableXactHashLock
Waiting to retrieve or store information about serializable transactions.
SerializableFinishedListLock
Waiting to access the list of finished serializable transactions.
SerializablePredicateLockListLock
Waiting to perform an operation on a list of locks held by serializable transactions.
OldSerXidLock
Waiting to read or record conflicting serializable transactions.
SyncRepLock
Waiting to read or update information about synchronous replicas.
BackgroundWorkerLock
Waiting to read or update background worker state.
DynamicSharedMemoryControlLock
Waiting to read or update dynamic shared memory state.
AutoFileLock
Waiting to update the postgresql.auto.conf file.
ReplicationSlotAllocationLock
Waiting to allocate or free a replication slot.
ReplicationSlotControlLock
Waiting to read or update replication slot state.
CommitTsControlLock
Waiting to read or update transaction commit timestamps.
CommitTsLock
Waiting to read or update the last value set for the transaction timestamp.
ReplicationOriginLock
Waiting to setup, drop or use replication origin.
MultiXactTruncationLock
Waiting to read or truncate multixact information.
OldSnapshotTimeMapLock
Waiting to read or update old snapshot control information.
2.2 LWLockTranche
clog
通常很少见，可能出现在在非常高并发的极小写事务时，文件IO出现等待，使用cgroup可以很容易复现。
Waiting for I/O on a clog (transaction status) buffer.
commit_timestamp
Waiting for I/O on commit timestamp buffer.
subtrans
Waiting for I/O a subtransaction buffer.
multixact_offset
Waiting for I/O on a multixact offset buffer.
multixact_member
Waiting for I/O on a multixact_member buffer.
async
Waiting for I/O on an async (notify) buffer.
oldserxid
Waiting to I/O on an oldserxid buffer.
wal_insert
Waiting to insert WAL into a memory buffer.
buffer_content
指 数据库 shared buffer
Waiting to read or write a data page in memory.
buffer_io
指 数据库 shared buffer
Waiting for I/O on a data page.
replication_origin
Waiting to read or update the replication progress.
replication_slot_io
Waiting for I/O on a replication slot.
proc
Waiting to read or update the fast-path lock information.
buffer_mapping
Waiting to associate a data block with a buffer in the buffer pool.
lock_manager
Waiting to add or examine locks for backends, or waiting to join or exit a locking group (used by parallel query).
predicate_lock_manager
Waiting to add or examine predicate lock information.
2.3 Lock
relation
Waiting to acquire a lock on a relation.
extend
Waiting to extend a relation.
page
Waiting to acquire a lock on page of a relation.
tuple
Waiting to acquire a lock on a tuple.
transactionid
Waiting for a transaction to finish.
virtualxid
Waiting to acquire a virtual xid lock.
speculative token
Waiting to acquire a speculative insertion lock.
object
Waiting to acquire a lock on a non-relation database object.
userlock
Waiting to acquire a userlock.
advisory
Waiting to acquire an advisory user lock.
2.4 BufferPin
BufferPin
Waiting to acquire a pin on a buffer.
3. 获取当指定PID当前的等待信息。
pg_stat_get_backend_wait_event_type(integer)
Wait event type name if backend is currently waiting, otherwise NULL. See Table 28-4 for details.
pg_stat_get_backend_wait_event(integer)
Wait event name if backend is currently waiting, otherwise NULL. See Table 28-4 for details.