sql - The initialization time of a query -


i'm new in query analyzing , understand concept mean. instance ran explain analyze query , got following row:

->  sort  (cost=816428.39..827478.32 rows=4419971 width=21)         (actual time=10780.477..12520.838 rows=4415703 loops=1)

as far google around it, in actual time=10780.477..12520.838 first number means time spent initialization, second 1 whole time spent part of query.

what initialization mean , why takes major part of execution

short: short answer question possible initiation time time taken find query execution plan + waiting locks.

elaboration: given in pg documentation, 2 numbers mentioned within 'actual times' are:

  • start time: time in millisecond when node started evaluate. (since haven't given entire explain analyse output, can't tell sure whether first node in execution plan). if first node evaluated, time time taken generate plan / time spent while waiting locks (if required) etc. however, if 'not' first node in execution plan, first millisecond when first data-item reached node, previous node in execution plan.

  • stop time: time in millisecond when node execution stopped. notably, second number 'not' time taken node, stop-time.

to elaborate take @ explain analyse output below:

# explain analyse update t set = 20 = 10;                                             query plan ---------------------------------------------------------------------------------------------------  update on t  (cost=0.00..40.00 rows=12 width=6) (actual time=5719.955..5719.955 rows=0 loops=1)    ->  seq scan on t  (cost=0.00..40.00 rows=12 width=6) (actual time=0.015..0.018 rows=1 loops=1)          filter: (a = 10)  total runtime: 5719.985 ms (4 rows)

although sequence scan went through fast, actualy update took time because in transaction locked row 5+ seconds... , update row took more 5000 milliseoncds (5 seconds) begin executing.


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