OushuDB 查看查询执行情况

使用\timing命令可以打印出查询执行的时间。

test=＃ \timing on

Timing is on.

test=＃ select * from foo; # 这时再执行SQL语句会给出语句执行时间。

  id | name

----+------

  1 | hawq

  2 | hdfs

(2 rows)

Time: 16.369 ms

test=＃ \timing off  # 关闭时间输出

Timing is off.

使用explain语句可以显示出查询计划。

test=＃ explain select count(*) from foo;

                                    QUERY PLAN

----------------------------------------------------------------------------------

  Aggregate  (cost=1.07..1.08 rows=1 width=8)

    ->  Gather Motion 1:1  (slice1; segments: 1)  (cost=1.03..1.06 rows=1 width=8)

      ->  Aggregate  (cost=1.03..1.04 rows=1 width=8)

            ->  Append-only Scan on foo  (cost=0.00..1.02 rows=2 width=0)

  Settings:  default_hash_table_bucket_number=6

(5 rows)

使用explain analyze可以显示出查询在具体执行时的状态，包括每一个操作符开始执行时间，以及结束时间，可以帮助用户找到查询的瓶颈，进而优化查询。关于查询计划以及explain analyze的执行结果的解释可以参考查询计划与查询执行章节。针对一个查询，可能会有无数个查询计划。得出优化的查询计划是查询优化器的功能。一个查询执行时间的长短与查询的计划有很大关系，所以熟悉查询计划以及具体查询的执行对查询优化有很大意义。

test=＃ explain analyze select count(*) from foo;

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Aggregate  (cost=1.07..1.08 rows=1 width=8)

Rows out:  Avg 1.0 rows x 1 workers.  Max/Last(seg-1:changlei/seg-1:changlei) 1/1 rows with 5.944/5.944 ms to end, start offset by 6.568/6.568 ms.

->  Gather Motion 1:1  (slice1; segments: 1)  (cost=1.03..1.06 rows=1 width=8)

      Rows out:  Avg 1.0 rows x 1 workers at destination.  Max/Last(seg-1:changlei/seg-1:changlei) 1/1 rows with 5.941/5.941 ms to first row, 5.942/5.942 ms to end, start offset by 6.569/6.569 ms.

      ->  Aggregate  (cost=1.03..1.04 rows=1 width=8)

            Rows out:  Avg 1.0 rows x 1 workers.  Max/Last(seg0:changlei/seg0:changlei) 1/1 rows with 5.035/5.035 ms to first row, 5.036/5.036 ms to end, start offset by 7.396/7.396 ms.

            ->  Append-only Scan on foo  (cost=0.00..1.02 rows=2 width=0)

                  Rows out:  Avg 2.0 rows x 1 workers.  Max/Last(seg0:changlei/seg0:changlei) 2/2 rows with 5.011/5.011 ms to first row, 5.032/5.032 ms to end, start offset by 7.397/7.397 ms.

Slice statistics:

  (slice0)    Executor memory: 223K bytes.

  (slice1)    Executor memory: 279K bytes (seg0:changlei).

Statement statistics:

  Memory used: 262144K bytes

Settings:  default_hash_table_bucket_number=6

Dispatcher statistics:

  executors used(total/cached/new connection): (1/1/0); dispatcher time(total/connection/dispatch data): (1.462 ms/0.000 ms/0.029 ms).

  dispatch data time(max/min/avg): (0.029 ms/0.029 ms/0.029 ms); consume executor data time(max/min/avg): (0.012 ms/0.012 ms/0.012 ms); free executor time(max/min/avg): (0.000 ms/0.000 ms/0.000 ms).

Data locality statistics:

  data locality ratio: 1.000; virtual segment number: 1; different host number: 1; virtual segment number per host(avg/min/max): (1/1/1); segment size(avg/min/max): (56.000 B/56 B/56 B); segment size with penalty(avg/min/max): (56.000 B/56 B/56 B); continuity(avg/min/max): (1.000/1.000/1.000); DFS metadatacache: 0.049 ms; resource allocation: 0.612 ms; datalocality calculation: 0.085 ms.

Total runtime: 13.398 ms

(20 rows)