update statistics

update statistics 是 SQL Server 中的一条命令，它用于更新数据表的统计信息，以便查询优化器可以更好地评估和选择执行计划。当 SQL Server 查询优化器需要为一个查询编制执行计划时，它会使用表的统计信息来预测查询的行数、数据分布和数据分页等信息，然后选择最优的执行计划。

update statistics 命令可以手动更新指定表的统计信息，以便保证查询优化器使用最新的信息进行查询优化和执行计划选择。可以使用 WITH FULLSCAN 选项更新所有行的统计信息，或者使用 WITH SAMPLE 选项更新部分行的统计信息。

需要注意的是，update statistics 命令可能会引起锁冲突和性能问题，因此在生产环境中应该谨慎使用，并在非高峰期进行操作。

相关问题

查看update语句执行进度

根据提供的引用内容，我们可以使用以下步骤来查看update语句的执行进度：

1. 确保TIMED_STATISTICS或SQL_TRACE参数已设置为true，这将启用SQL跟踪功能，以便我们可以查看SQL语句的执行进度。

2. 执行update语句，并获取其SQL标识符。

3. 使用以下命令来查看SQL语句的执行进度：

   SELECT sofar, totalwork, round(sofar/totalwork*100,2) "% Complete"
   FROM v$session_longops
   WHERE opname LIKE 'UPDATE%' AND target_desc = '<SQL_IDENTIFIER>';

其中，<SQL_IDENTIFIER>是我们在第2步中获取的SQL标识符。

该命令将返回一个结果集，其中包含sofar（已完成的工作量），totalwork（总工作量）和完成百分比（"% Complete"）等列。通过查看完成百分比列，我们可以了解update语句的执行进度。

例如，如果我们的update语句的SQL标识符为12345，则可以使用以下命令来查看其执行进度：

   SELECT sofar, totalwork, round(sofar/totalwork*100,2) "% Complete"
   FROM v$session_longops
   WHERE opname LIKE 'UPDATE%' AND target_desc = '12345';

[0710/081418.767757] [INFO:main.cc(55)] A/B Update Engine starting [0710/081418.775873] [INFO:boot_control_android.cc(69)] Loaded boot control hidl hal. [0710/081418.844522] [ERROR:update_attempter_android.cc(936)] prefs_->GetInt64(kPrefsPreviousSlot, &previous_slot) failed. [0710/081418.850761] [INFO:update_attempter_android.cc(196)] OTAResult::NOT_ATTEMPTED [0710/081418.883202] [INFO:update_attempter_android.cc(1262)] Scheduling CleanupPreviousUpdateAction. [0710/081418.895190] [INFO:action_processor.cc(51)] ActionProcessor: starting CleanupPreviousUpdateAction [0710/081418.899394] [INFO:cleanup_previous_update_action.cc(149)] Starting/resuming CleanupPreviousUpdateAction [0710/081418.902350] [INFO:cleanup_previous_update_action.cc(124)] CleanupPreviousUpdateAction scheduled task ID 1 for WaitBootCompleted [0710/081420.904474] [INFO:cleanup_previous_update_action.cc(112)] Executing task 1 [0710/081420.906355] [INFO:cleanup_previous_update_action.cc(124)] CleanupPreviousUpdateAction scheduled task ID 3 for WaitBootCompleted [0710/081422.908519] [INFO:cleanup_previous_update_action.cc(112)] Executing task 3 [0710/081422.910627] [INFO:cleanup_previous_update_action.cc(124)] CleanupPreviousUpdateAction scheduled task ID 4 for WaitBootCompleted [0710/081424.915286] [INFO:cleanup_previous_update_action.cc(112)] Executing task 4 [0710/081424.919637] [INFO:cleanup_previous_update_action.cc(124)] CleanupPreviousUpdateAction scheduled task ID 5 for WaitBootCompleted [0710/081426.923179] [INFO:cleanup_previous_update_action.cc(112)] Executing task 5 [0710/081426.927580] [INFO:cleanup_previous_update_action.cc(124)] CleanupPreviousUpdateAction scheduled task ID 6 for WaitBootCompleted [0710/081428.930648] [INFO:cleanup_previous_update_action.cc(112)] Executing task 6 [0710/081428.936232] [INFO:cleanup_previous_update_action.cc(124)] CleanupPreviousUpdateAction scheduled task ID 7 for WaitBootCompleted [0710/081430.938563] [INFO:cleanup_previous_update_action.cc(112)] Executing task 7 [0710/081430.943791] [INFO:cleanup_previous_update_action.cc(189)] Boot completed, waiting on markBootSuccessful() [0710/081430.947213] [INFO:snapshot.cpp(3766)] EnsureMetadataMounted does nothing in Android mode. [0710/081430.950810] [INFO:snapshot_stats.cpp(37)] Read merge statistics file failed: No such file or directory [0710/081430.965136] [INFO:cleanup_previous_update_action.cc(261)] Waiting for any previous merge request to complete. This can take up to several minutes. [0710/081430.974341] [INFO:snapshot.cpp(1047)] CheckMergeState for snapshots returned: 0 [0710/081430.981751] [INFO:snapshot.cpp(1019)] ProcessUpdateState handling state: 0 [0710/081430.988898] [INFO:cleanup_previous_update_action.cc(297)] Can't find any snapshot to merge. [0710/081430.996749] [INFO:cleanup_previous_update_action.cc(130)] Stopping/suspending/completing CleanupPreviousUpdateAction [0710/081431.002327] [INFO:cleanup_previous_update_action.cc(479)] Not reporting merge stats because state is None [0710/081431.014705] [INFO:cleanup_previous_update_action.cc(130)] Stopping/suspending/completing CleanupPreviousUpdateAction [0710/081431.022443] [INFO:action_processor.cc(116)] ActionProcessor: finished last action CleanupPreviousUpdateAction with code ErrorCode::kSuccess [0710/081431.031005] [INFO:update_attempter_android.cc(570)] Processing Done. [0710/081431.038024] [INFO:update_attempter_android.cc(826)] Clearing update complete marker. [0710/081431.046821] [INFO:update_attempter_android.cc(722)] Terminating cleanup previous update. 什么意思？

这段日志是关于一个名为 "CleanupPreviousUpdateAction" 的操作的记录。根据日志，它是在 Android 系统中执行的，目的是清理之前的更新操作。

具体来说，日志中显示了一系列任务被执行，这些任务被安排在启动完成后执行。任务的目标是等待系统启动完成，并在特定任务执行之前执行清理操作。

但是，在这个日志中，没有找到任何需要合并的快照（snapshot），因此没有进行任何合并操作。

最后，日志显示 "CleanupPreviousUpdateAction" 操作已经完成，并以成功的状态结束。

所以，总的来说，这段日志是关于清理之前更新操作的记录，并且没有进行任何合并操作。