28.2. 統計資訊收集器

版本：11

PostgreSQL 的統計資訊收集器是一個子系統，支援收集和回報有關伺服器活動的訊息。目前，收集器可以計算對磁盤區塊和單個資料列中資料表和索引的存取。它還追踪每個資料表中的總資料列數量，以及有關每個資料表的清理和分析操作的資訊。它還可以統計對使用者定義函數的呼叫以及每個函數所花費的總時間。

PostgreSQL 也支援回報有關系統當下正在發生什麼的動態訊息，例如目前正由其他伺服器程序所執行的明確指令，以及系統中正在進行的其他連線。此功能獨立於收集器程序。

28.2.1. 統計收集設定

由於統計信息的收集會增加查詢執行的成本，因此系統可以設定收集或不收集訊息。這由通常在 postgresql.conf 中的參數控制。（有關設定參數的詳細訊息，請參閱第 19 章。）

參數 track_activities 啓用監控任何伺服器程序正在執行的指令。

參數 track_counts 控制是否收集有關資料表和索引存取的統計資訊。

參數 track_functions 可以追踪使用者自訂函數的使用情況。

參數 track_io_timing 啓用監控磁碟區塊讀取和寫入時間。

通常這些參數會在 postgresql.conf 中設定，以便它們適用於所有伺服器程序，但也可以使用 SET 指令在各個連線中啓用或停用它們。（為了防止普通使用者將其活動隱藏於管理員之外，只允許超級使用者使用 SET 變更這些參數。）

統計資訊收集器透過臨時檔案將收集的資訊傳輸到其他 PostgreSQL 程序。這些檔案儲存在參數 stats_temp_directory 所指定的目錄中，預設為 pg_stat_tmp。為了獲得更好的效能，可以將 stats_temp_directory 指向記憶體的檔案系統，從而降低物理 I/O 需求。當伺服器完全地關閉時，統計資料的永久副本將儲存在 pg_stat 子目錄中，以便可以跨伺服器重新啟動並保留統計資訊。在伺服器啟動時進行回復（例如，在立即關閉，伺服器當機和實時回復之後），將重置所有統計計數器。

28.2.2. Viewing Statistics

Table 28.1 中列出了的幾個預先定義好的檢視表，可用於顯示系統的當下的狀態。Table 28.2 中還列出了其他幾個檢視表，可用於顯示統計訊息收集的結果。進一步可以使用基礎統計函數建構自訂的檢視表，如第 28.2.3 節中所述。

使用統計訊息監控收集的資料時，重要的是要瞭解到訊息並不會立即更新。每個伺服器程序在回到閒置狀態之前會向收集器發送新的統計計數；因此，仍在進行中的查詢或事務不會對顯示的數據產生影響。此外，收集器本身每 PGSTAT_STAT_INTERVAL 毫秒最多回報一次（500毫秒，除非在編譯伺服器時更改）。因此顯示的數據是落後於實際活動的。 但是，track_activities 所收集的目前查詢資訊是最新的。

另一個要點是，當要求伺服器程序顯示任何這些統計訊息時，它首先獲取收集器程序發出的最新報告，然後繼續將此快照結果用於所有統計檢視表和函數，直到其當下該事務結束。因此，只要您繼續該事務，統計數據就會顯示靜態結果。同樣地，當在事務中首次請求任何此類資訊時，收集關於所有連線的當下查詢資訊，將會在整個事務中顯示相同的資訊。這是一個正常的功能，而不是一個錯誤，因為它允許您對統計信息執行多個查詢並且關聯結果，而不必擔心數字在您交易當下發生變化。但是，如果要查看每個查詢的新結果，請確保在任何事務區塊之外執行查詢。或者，您可以呼叫 pg_stat_clear_snapshot()，它將丟棄當下事務的統計資訊快照（如果有的話）。下次使用統計資訊時將會去獲取新的快照。

事務還可以在檢視圖 pg_stat_xact_all_tables，pg_stat_xact_sys_tables，pg_stat_xact_user_tables 和pg_stat_xact_user_functions 中查看自己的統計訊息（尚未傳遞給收集器的）。 這些數字不符合上述原則；相反地，他們在整個交易過程中不斷更新。

Table 28.1. Dynamic Statistics Views

View Name	Description
pg_stat_activity	每個伺服器程序一個資料列，顯示與該程序的當時活動相關的訊息，例如狀態和當時查詢。有關詳細訊息，請參閱 pg_stat_activity。
pg_stat_replication	One row per WAL sender process, showing statistics about replication to that sender's connected standby server. See pg_stat_replication for details.
pg_stat_wal_receiver	Only one row, showing statistics about the WAL receiver from that receiver's connected server. See pg_stat_wal_receiver for details.
pg_stat_subscription	At least one row per subscription, showing information about the subscription workers. See pg_stat_subscription for details.
pg_stat_ssl	One row per connection (regular and replication), showing information about SSL used on this connection. See pg_stat_ssl for details.
pg_stat_progress_vacuum	One row for each backend (including autovacuum worker processes) running VACUUM, showing current progress. See Section 28.4.1.

Table 28.2. Collected Statistics Views

View Name	Description
pg_stat_archiver	One row only, showing statistics about the WAL archiver process's activity. See pg_stat_archiver for details.
pg_stat_bgwriter	One row only, showing statistics about the background writer process's activity. See pg_stat_bgwriter for details.
pg_stat_database	One row per database, showing database-wide statistics. See pg_stat_database for details.
pg_stat_database_conflicts	One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See pg_stat_database_conflicts for details.
pg_stat_all_tables	One row for each table in the current database, showing statistics about accesses to that specific table. See pg_stat_all_tables for details.
pg_stat_sys_tables	Same as pg_stat_all_tables, except that only system tables are shown.
pg_stat_user_tables	Same as pg_stat_all_tables, except that only user tables are shown.
pg_stat_xact_all_tables	Similar to pg_stat_all_tables, but counts actions taken so far within the current transaction (which are not yet included in pg_stat_all_tables and related views). The columns for numbers of live and dead rows and vacuum and analyze actions are not present in this view.
pg_stat_xact_sys_tables	Same as pg_stat_xact_all_tables, except that only system tables are shown.
pg_stat_xact_user_tables	Same as pg_stat_xact_all_tables, except that only user tables are shown.
pg_stat_all_indexes	One row for each index in the current database, showing statistics about accesses to that specific index. See pg_stat_all_indexes for details.
pg_stat_sys_indexes	Same as pg_stat_all_indexes, except that only indexes on system tables are shown.
pg_stat_user_indexes	Same as pg_stat_all_indexes, except that only indexes on user tables are shown.
pg_statio_all_tables	One row for each table in the current database, showing statistics about I/O on that specific table. See pg_statio_all_tables for details.
pg_statio_sys_tables	Same as pg_statio_all_tables, except that only system tables are shown.
pg_statio_user_tables	Same as pg_statio_all_tables, except that only user tables are shown.
pg_statio_all_indexes	One row for each index in the current database, showing statistics about I/O on that specific index. See pg_statio_all_indexes for details.
pg_statio_sys_indexes	Same as pg_statio_all_indexes, except that only indexes on system tables are shown.
pg_statio_user_indexes	Same as pg_statio_all_indexes, except that only indexes on user tables are shown.
pg_statio_all_sequences	One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See pg_statio_all_sequences for details.
pg_statio_sys_sequences	Same as pg_statio_all_sequences, except that only system sequences are shown. (Presently, no system sequences are defined, so this view is always empty.)
pg_statio_user_sequences	Same as pg_statio_all_sequences, except that only user sequences are shown.
pg_stat_user_functions	One row for each tracked function, showing statistics about executions of that function. See pg_stat_user_functions for details.
pg_stat_xact_user_functions	Similar to pg_stat_user_functions, but counts only calls during the current transaction (which are not yet included in pg_stat_user_functions).

The per-index statistics are particularly useful to determine which indexes are being used and how effective they are.

The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. When the number of actual disk reads is much smaller than the number of buffer hits, then the cache is satisfying most read requests without invoking a kernel call. However, these statistics do not give the entire story: due to the way in which PostgreSQL handles disk I/O, data that is not in the PostgreSQL buffer cache might still reside in the kernel's I/O cache, and might therefore still be fetched without requiring a physical read. Users interested in obtaining more detailed information on PostgreSQL I/O behavior are advised to use the PostgreSQL statistics collector in combination with operating system utilities that allow insight into the kernel's handling of I/O.

Table 28.3. pg_stat_activity View

Column	Type	Description
datid	oid	此後端連線到的資料庫 OID
datname	name	此後端連線到的資料庫名稱
pid	integer	此後端的程序 ID
usesysid	oid	登入此後端的使用者 OID
usename	name	登入到此後端的使用者名稱
application_name	text	連線到此後端的應用程序名稱
client_addr	inet	連線到此後端的客戶端的 IP 位址。如果此字串為空，則表示客戶端通過伺服器上的 Unix socket 連線，或者這是內部程序（如 autovacuum）。
client_hostname	text	已連接用戶端的主機名稱，由 client_addr 的反向 DNS 查詢回報。對於 IP 連線，只有在 log_hostname 啓用時，此字串才不是空字串。
client_port	integer	用戶端用於與此後端通訊的 TCP 連接埠號碼，如果使用 Unix domain socket，則為 -1
backend_start	timestamp with time zone	此程序開始的時間。對於用戶端後端，這是用戶端連線到伺服器的時間。
xact_start	timestamp with time zone	此程序的當下交易事務已啟動的時間，如果沒有交易事務處於活動狀態，則為null。如果當下查詢是其交易事務的第一個，則此欄位等於 query_start 欄位。
query_start	timestamp with time zone	最後一個查詢啟動時，當下活動查詢的啟動時間，或狀態是否處於活動狀態
state_change	timestamp with time zone	狀態最後一次變更的時間
wait_event_type	text	The type of event for which the backend is waiting, if any; otherwise NULL. Possible values are:LWLock: The backend is waiting for a lightweight lock. Each such lock protects a particular data structure in shared memory. wait_event will contain a name identifying the purpose of the lightweight lock. (Some locks have specific names; others are part of a group of locks each with a similar purpose.)Lock: 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. wait_event will identify the type of lock awaited.BufferPin: 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.Activity: The server process is idle. This is used by system processes waiting for activity in their main processing loop. wait_event will identify the specific wait point.Extension: The server process is waiting for activity in an extension module. This category is useful for modules to track custom waiting points.Client: The server process is waiting for some activity on a socket from user applications, and that the server expects something to happen that is independent from its internal processes. wait_event will identify the specific wait point.IPC: The server process is waiting for some activity from another process in the server. wait_event will identify the specific wait point.Timeout: The server process is waiting for a timeout to expire. wait_event will identify the specific wait point.IO: The server process is waiting for a IO to complete. wait_event will identify the specific wait point.
wait_event	text	Wait event name if backend is currently waiting, otherwise NULL. See Table 28.4 for details.
state	text	Current overall state of this backend. Possible values are:active: The backend is executing a query.idle: The backend is waiting for a new client command.idle in transaction: The backend is in a transaction, but is not currently executing a query.idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error.fastpath function call: The backend is executing a fast-path function.disabled: This state is reported if track_activities is disabled in this backend.
backend_xid	xid	Top-level transaction identifier of this backend, if any.
backend_xmin	xid	The current backend's xmin horizon.
query	text	Text of this backend's most recent query. If state is active this field shows the currently executing query. In all other states, it shows the last query that was executed. By default the query text is truncated at 1024 characters; this value can be changed via the parameter track_activity_query_size.
backend_type	text	當下後端的類型。可能的類型是 autovacuum launcher, autovacuum worker, background worker, background writer, client backend, checkpointer, startup, walreceiver, walsender and walwriter.

The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process.

Note

The wait_event and state columns are independent. If a backend is in the active state, it may or may not be waiting on some event. If the state is active and wait_event is non-null, it means that a query is being executed, but is being blocked somewhere in the system.

Table 28.4. wait_event Description

Wait Event Type	Wait Event Name	Description
LWLock	ShmemIndexLock	Waiting to find or allocate space in shared memory.
OidGenLock	Waiting to allocate or assign an OID.
XidGenLock	Waiting to allocate or assign a transaction id.
ProcArrayLock	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	Waiting for WAL buffers to be written to disk.
ControlFileLock	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	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	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	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.
BackendRandomLock	Waiting to generate a random number.
LogicalRepWorkerLock	Waiting for action on logical replication worker to finish.
CLogTruncationLock	Waiting to truncate the write-ahead log or waiting for write-ahead log truncation to finish.
clog	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	Waiting to read or write a data page in memory.
buffer_io	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.
parallel_query_dsa	Waiting for parallel query dynamic shared memory allocation lock.
tbm	Waiting for TBM shared iterator lock.
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 user lock.
advisory	Waiting to acquire an advisory user lock.
BufferPin	BufferPin	Waiting to acquire a pin on a buffer.
Activity	ArchiverMain	Waiting in main loop of the archiver process.
AutoVacuumMain	Waiting in main loop of autovacuum launcher process.
BgWriterHibernate	Waiting in background writer process, hibernating.
BgWriterMain	Waiting in main loop of background writer process background worker.
CheckpointerMain	Waiting in main loop of checkpointer process.
LogicalLauncherMain	Waiting in main loop of logical launcher process.
LogicalApplyMain	Waiting in main loop of logical apply process.
PgStatMain	Waiting in main loop of the statistics collector process.
RecoveryWalAll	Waiting for WAL from any kind of source (local, archive or stream) at recovery.
RecoveryWalStream	Waiting for WAL from a stream at recovery.
SysLoggerMain	Waiting in main loop of syslogger process.
WalReceiverMain	Waiting in main loop of WAL receiver process.
WalSenderMain	Waiting in main loop of WAL sender process.
WalWriterMain	Waiting in main loop of WAL writer process.
Client	ClientRead	Waiting to read data from the client.
ClientWrite	Waiting to write data from the client.
LibPQWalReceiverConnect	Waiting in WAL receiver to establish connection to remote server.
LibPQWalReceiverReceive	Waiting in WAL receiver to receive data from remote server.
SSLOpenServer	Waiting for SSL while attempting connection.
WalReceiverWaitStart	Waiting for startup process to send initial data for streaming replication.
WalSenderWaitForWAL	Waiting for WAL to be flushed in WAL sender process.
WalSenderWriteData	Waiting for any activity when processing replies from WAL receiver in WAL sender process.
Extension	Extension	Waiting in an extension.
IPC	BgWorkerShutdown	Waiting for background worker to shut down.
BgWorkerStartup	Waiting for background worker to start up.
BtreePage	Waiting for the page number needed to continue a parallel B-tree scan to become available.
ExecuteGather	Waiting for activity from child process when executing Gather node.
LogicalSyncData	Waiting for logical replication remote server to send data for initial table synchronization.
LogicalSyncStateChange	Waiting for logical replication remote server to change state.
MessageQueueInternal	Waiting for other process to be attached in shared message queue.
MessageQueuePutMessage	Waiting to write a protocol message to a shared message queue.
MessageQueueReceive	Waiting to receive bytes from a shared message queue.
MessageQueueSend	Waiting to send bytes to a shared message queue.
ParallelFinish	Waiting for parallel workers to finish computing.
ParallelBitmapScan	Waiting for parallel bitmap scan to become initialized.
ProcArrayGroupUpdate	Waiting for group leader to clear transaction id at transaction end.
ReplicationOriginDrop	Waiting for a replication origin to become inactive to be dropped.
ReplicationSlotDrop	Waiting for a replication slot to become inactive to be dropped.
SafeSnapshot	Waiting for a snapshot for a READ ONLY DEFERRABLE transaction.
SyncRep	Waiting for confirmation from remote server during synchronous replication.
Timeout	BaseBackupThrottle	Waiting during base backup when throttling activity.
PgSleep	Waiting in process that called pg_sleep.
RecoveryApplyDelay	Waiting to apply WAL at recovery because it is delayed.
IO	BufFileRead	Waiting for a read from a buffered file.
BufFileWrite	Waiting for a write to a buffered file.
ControlFileRead	Waiting for a read from the control file.
ControlFileSync	Waiting for the control file to reach stable storage.
ControlFileSyncUpdate	Waiting for an update to the control file to reach stable storage.
ControlFileWrite	Waiting for a write to the control file.
ControlFileWriteUpdate	Waiting for a write to update the control file.
CopyFileRead	Waiting for a read during a file copy operation.
CopyFileWrite	Waiting for a write during a file copy operation.
DataFileExtend	Waiting for a relation data file to be extended.
DataFileFlush	Waiting for a relation data file to reach stable storage.
DataFileImmediateSync	Waiting for an immediate synchronization of a relation data file to stable storage.
DataFilePrefetch	Waiting for an asynchronous prefetch from a relation data file.
DataFileRead	Waiting for a read from a relation data file.
DataFileSync	Waiting for changes to a relation data file to reach stable storage.
DataFileTruncate	Waiting for a relation data file to be truncated.
DataFileWrite	Waiting for a write to a relation data file.
DSMFillZeroWrite	Waiting to write zero bytes to a dynamic shared memory backing file.
LockFileAddToDataDirRead	Waiting for a read while adding a line to the data directory lock file.
LockFileAddToDataDirSync	Waiting for data to reach stable storage while adding a line to the data directory lock file.
LockFileAddToDataDirWrite	Waiting for a write while adding a line to the data directory lock file.
LockFileCreateRead	Waiting to read while creating the data directory lock file.
LockFileCreateSync	Waiting for data to reach stable storage while creating the data directory lock file.
LockFileCreateWrite	Waiting for a write while creating the data directory lock file.
LockFileReCheckDataDirRead	Waiting for a read during recheck of the data directory lock file.
LogicalRewriteCheckpointSync	Waiting for logical rewrite mappings to reach stable storage during a checkpoint.
LogicalRewriteMappingSync	Waiting for mapping data to reach stable storage during a logical rewrite.
LogicalRewriteMappingWrite	Waiting for a write of mapping data during a logical rewrite.
LogicalRewriteSync	Waiting for logical rewrite mappings to reach stable storage.
LogicalRewriteWrite	Waiting for a write of logical rewrite mappings.
RelationMapRead	Waiting for a read of the relation map file.
RelationMapSync	Waiting for the relation map file to reach stable storage.
RelationMapWrite	Waiting for a write to the relation map file.
ReorderBufferRead	Waiting for a read during reorder buffer management.
ReorderBufferWrite	Waiting for a write during reorder buffer management.
ReorderLogicalMappingRead	Waiting for a read of a logical mapping during reorder buffer management.
ReplicationSlotRead	Waiting for a read from a replication slot control file.
ReplicationSlotRestoreSync	Waiting for a replication slot control file to reach stable storage while restoring it to memory.
ReplicationSlotSync	Waiting for a replication slot control file to reach stable storage.
ReplicationSlotWrite	Waiting for a write to a replication slot control file.
SLRUFlushSync	Waiting for SLRU data to reach stable storage during a checkpoint or database shutdown.
SLRURead	Waiting for a read of an SLRU page.
SLRUSync	Waiting for SLRU data to reach stable storage following a page write.
SLRUWrite	Waiting for a write of an SLRU page.
SnapbuildRead	Waiting for a read of a serialized historical catalog snapshot.
SnapbuildSync	Waiting for a serialized historical catalog snapshot to reach stable storage.
SnapbuildWrite	Waiting for a write of a serialized historical catalog snapshot.
TimelineHistoryFileSync	Waiting for a timeline history file received via streaming replication to reach stable storage.
TimelineHistoryFileWrite	Waiting for a write of a timeline history file received via streaming replication.
TimelineHistoryRead	Waiting for a read of a timeline history file.
TimelineHistorySync	Waiting for a newly created timeline history file to reach stable storage.
TimelineHistoryWrite	Waiting for a write of a newly created timeline history file.
TwophaseFileRead	Waiting for a read of a two phase state file.
TwophaseFileSync	Waiting for a two phase state file to reach stable storage.
TwophaseFileWrite	Waiting for a write of a two phase state file.
WALBootstrapSync	Waiting for WAL to reach stable storage during bootstrapping.
WALBootstrapWrite	Waiting for a write of a WAL page during bootstrapping.
WALCopyRead	Waiting for a read when creating a new WAL segment by copying an existing one.
WALCopySync	Waiting a new WAL segment created by copying an existing one to reach stable storage.
WALCopyWrite	Waiting for a write when creating a new WAL segment by copying an existing one.
WALInitSync	Waiting for a newly initialized WAL file to reach stable storage.
WALInitWrite