# Maintenance Command
Persist all the data points in the memory table of the storage group to the disk, and seal the data file.
Note: This command does not need to be invoked manually by the client. IoTDB has WAL to ensure data security and IoTDB will flush when appropriate. Frequently call flush can result in small data files that degrade query performance.
IoTDB> FLUSH IoTDB> FLUSH root.ln IoTDB> FLUSH root.sg1,root.sg2
Merge sequence and unsequence data. Currently IoTDB supports the following two types of SQL to manually trigger the merge process of data files:
MERGEOnly rewrite overlapped Chunks, the merge speed is quick, while there will be redundant data on the disk eventually.
FULL MERGERewrite all data in overlapped files, the merge speed is slow, but there will be no redundant data on the disk eventually.
IoTDB> MERGE IoTDB> FULL MERGE
# CLEAR CACHE
Clear the cache of chunk, chunk metadata and timeseries metadata to release the memory footprint.
IoTDB> CLEAR CACHE
# SET STSTEM TO READONLY / WRITABLE
Manually set IoTDB system to read-only or writable mode.
IoTDB> SET SYSTEM TO READONLY IoTDB> SET SYSTEM TO WRITABLE
# SCHEMA SNAPSHOT
To speed up restarting of IoTDB, users can create snapshot of schema and avoid recovering schema from mlog file.
IoTDB> CREATE SNAPSHOT FOR SCHEMA
# Kill Query
When using IoTDB, you may encounter the following situations: you have entered a query statement, but can not get the result for a long time, as this query contains too much data or some other reasons, and have to wait until the query ends. Since version 0.12, IoTDB has provided two solutions for queries with long execution time: query timeout and query abort.
# Query timeout
For queries that take too long to execute, IoTDB will forcibly interrupt the query and throw a timeout exception, as shown in the figure:
IoTDB> select * from root; Msg: 701 Current query is time out, please check your statement or modify timeout parameter.
The default timeout of the system is 60000 ms，which can be customized in the configuration file through the
If you use JDBC or Session, we also support setting a timeout for a single query（Unit: ms）：
((IoTDBStatement) statement).executeQuery(String sql, long timeoutInMS) session.executeQueryStatement(String sql, long timeout)
If the timeout parameter is not configured or with value 0, the default timeout time will be used.
# Query abort
In addition to waiting for the query to time out passively, IoTDB also supports stopping the query actively:
KILL QUERY <queryId>
You can abort the specified query by specifying
queryId is not specified, all executing queries will be killed.
To get the executing
queryId，you can use the
show query processlist command，which will show the list of all executing queries，with the following result set：
The maximum display length of statement is 64 characters. For statements with more than 64 characters, the intercepted part will be displayed.
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