我的系统在实际应用中当show processlist的时候看到大多时候都是Sending data!也就是我一直非常奇怪为什么会有这么多Sending data 滞留在我的系统中,真是往客户端发送查询结果不应该这么慢的!
文档上说:
我特别想要说明的是黄底部分的,虽然字面上看起来好像这个状态意味着在给客户端发送数据而已,实际上不完全是这样的,比如MySQL找到查询结果(30条记录,没有Cache到内存中,散布在硬盘上不同的地方)了,这样的话实际上要从这30个不同的地方取出这30条记录,也就是要寻道30次(特别散,Cache不到 :)),这个任务我觉得可以看成 The thread is processing rows for a
这个时候也许要想办法去增加内存,让MySQL不但将Index能Cache到内存中,才能够更多地将具体数据Cache到内存中了!
来自MySQL手册:
The following list describes thread
文档上说:
Sending data
The thread is processing rows for a SELECT
statement and is also sending data to the client.我特别想要说明的是黄底部分的,虽然字面上看起来好像这个状态意味着在给客户端发送数据而已,实际上不完全是这样的,比如MySQL找到查询结果(30条记录,没有Cache到内存中,散布在硬盘上不同的地方)了,这样的话实际上要从这30个不同的地方取出这30条记录,也就是要寻道30次(特别散,Cache不到 :)),这个任务我觉得可以看成 The thread is processing rows for a
SELECT
statement !这个时候也许要想办法去增加内存,让MySQL不但将Index能Cache到内存中,才能够更多地将具体数据Cache到内存中了!
来自MySQL手册:
The following list describes thread
State
values
that are associated with general query processing and not more specialized
activities such as replication. Many of these are useful only for finding bugs
in the server. -
After create
Occurs when the thread creates a table (including internal temporary tables), at the end of the function that creates the table. This state is used even if the table could not be created due to some error.
-
Analyzing
The thread is calculating aMyISAM
table key distributions (for example, forANALYZE TABLE
).
-
Checking table
The thread is performing a table check operation.
-
cleaning up
The thread has processed one command and is preparing to free memory and reset certain state variables.
-
closing tables
Means that the thread is flushing the changed table data to disk and closing the used tables. This should be a fast operation. If not, you should verify that you do not have a full disk and that the disk is not in very heavy use.
-
converting HEAP to MyISAM
The thread is converting an internal temporary table from aMEMORY
table to an on-diskMyISAM
table.
-
copy to tmp table
The thread is processing anALTER TABLE
statement. This state occurs after the table with the new structure has been created but before rows are copied into it.
-
Copying to group table
If a statement has differentORDER BY
andGROUP BY
criteria, the rows are sorted by group and copied to a temporary table.
-
Copying to tmp table
The server is copying to a temporary table in memory.
-
Copying to tmp table on disk
The server is copying to a temporary table on disk. The temporary result set was larger thantmp_table_size
and the thread is changing the temporary table from in-memory to disk-based format to save memory.
-
Creating index
The thread is processingALTER TABLE ... ENABLE KEYS
for aMyISAM
table.
-
Creating sort index
The thread is processing aSELECT
that is resolved using an internal temporary table.
-
creating table
The thread is creating a table. This includes creation of temporary tables.
-
Creating tmp table
The thread is creating a temporary table in memory or on disk. If the table is created in memory but later is converted to an on-disk table, the state during that operation will beCopying to tmp table on disk
.
-
deleting from main table
The server is executing the first part of a multiple-table delete. It is deleting only from the first table, and saving fields and offsets to be used for deleting from the other (reference) tables.
-
deleting from reference tables
The server is executing the second part of a multiple-table delete and deleting the matched rows from the other tables.
-
discard_or_import_tablespace
The thread is processing anALTER TABLE ... DISCARD TABLESPACE
orALTER TABLE ... IMPORT TABLESPACE
statement.
-
end
This occurs at the end but before the cleanup ofALTER TABLE
,CREATE VIEW
,DELETE
,INSERT
,SELECT
, orUPDATE
statements.
-
Execution of init_command
The thread is executing statements in the value of theinit_command
system variable.
-
freeing items
The thread has executed a command. This state is usually followed bycleaning up
.
-
Flushing tables
The thread is executingFLUSH TABLES
and is waiting for all threads to close their tables.
-
FULLTEXT initialization
The server is preparing to perform a natural-language full-text search.
-
init
This occurs before the initialization ofALTER TABLE
,DELETE
,INSERT
,SELECT
, orUPDATE
statements.
-
Killed
Someone has sent aKILL
statement to the thread and it should abort next time it checks the kill flag. The flag is checked in each major loop in MySQL, but in some cases it might still take a short time for the thread to die. If the thread is locked by some other thread, the kill takes effect as soon as the other thread releases its lock.
-
Locked
The query is locked by another query.
-
logging slow query
The thread is writing a statement to the slow-query log.
-
login
The initial state for a connection thread until the client has been authenticated successfully.
-
Opening tables
,Opening table
The thread is trying to open a table. This is should be very fast procedure, unless something prevents opening. For example, anALTER TABLE
or aLOCK TABLE
statement can prevent opening a table until the statement is finished.
-
preparing
This state occurs during query optimization.
-
Purging old relay logs
The thread is removing unneeded relay log files.
-
query end
This state occurs after processing a query but before thefreeing items
state.
-
Reading from net
The server is reading a packet from the network.
-
Removing duplicates
The query was usingSELECT DISTINCT
in such a way that MySQL could not optimize away the distinct operation at an early stage. Because of this, MySQL requires an extra stage to remove all duplicated rows before sending the result to the client.
-
removing tmp table
The thread is removing an internal temporary table after processing aSELECT
statement. This state is not used if no temporary table was created.
-
rename
The thread is renaming a table.
-
rename result table
The thread is processing anALTER TABLE
statement, has created the new table, and is renaming it to replace the original table.
-
Reopen tables
The thread got a lock for the table, but noticed after getting the lock that the underlying table structure changed. It has freed the lock, closed the table, and is trying to reopen it.
-
Repair by sorting
The repair code is using a sort to create indexes.
-
Repair done
The thread has completed a multi-threaded repair for aMyISAM
table.
-
Repair with keycache
The repair code is using creating keys one by one through the key cache. This is much slower thanRepair by sorting
.
-
Rolling back
The thread is rolling back a transaction.
-
Saving state
ForMyISAM
table operations such as repair or analysis, the thread is saving the new table state to the.MYI
file header. State includes information such as number of rows, theAUTO_INCREMENT
counter, and key distributions.
-
Searching rows for update
The thread is doing a first phase to find all matching rows before updating them. This has to be done if theUPDATE
is changing the index that is used to find the involved rows.
-
Sending data
The thread is processing rows for aSELECT
statement and also is sending data to the client.
-
setup
The thread is beginning anALTER TABLE
operation.
-
Sorting for group
The thread is doing a sort to satisfy aGROUP BY
.
-
Sorting for order
The thread is doing a sort to satisfy aORDER BY
.
-
Sorting index
The thread is sorting index pages for more efficient access during aMyISAM
table optimization operation.
-
Sorting result
For aSELECT
statement, this is similar toCreating sort index
, but for non-temporary tables.
-
statistics
The server is calculating statistics to develop a query execution plan.
-
System lock
The thread is going to request or is waiting for an external system lock for the table. If you are not using multiple mysqld servers that are accessing the same tables, you can disable system locks with the--skip-external-locking
option.
-
Table lock
The next thread state afterSystem lock
. The thread has acquired an external lock and is going to request an internal table lock.
-
Updating
The thread is searching for rows to update and is updating them.
-
updating main table
The server is executing the first part of a multiple-table update. It is updating only the first table, and saving fields and offsets to be used for updating the other (reference) tables.
-
updating reference tables
The server is executing the second part of a multiple-table update and updating the matched rows from the other tables.
-
User lock
The thread is going to request or is waiting for an advisory lock requested with aGET_LOCK()
call.
-
Waiting for tables
,Waiting for table
The thread got a notification that the underlying structure for a table has changed and it needs to reopen the table to get the new structure. However, to reopen the table, it must wait until all other threads have closed the table in question.
This notification takes place if another thread has usedFLUSH TABLES
or one of the following statements on the table in question:FLUSH TABLES
,tbl_name
ALTER TABLE
,RENAME TABLE
,REPAIR TABLE
,ANALYZE TABLE
, orOPTIMIZE TABLE
.
-
Waiting on cond
A generic state in which the the thread is waiting for a condition to become true. No specific state information is available.
-
Writing to net
The server is writing a packet to the network