Do not do SELECT count(*) FROM ... if pagination is not requested

[dmugtasimov]

Even if I do not request pagination in HTTP-query LimitOffsetPagination still calls self.get_count(queryset) which results in an extra SELECT count(*) FROM .... This becomes important if the query itself is comparatively heavy. Let's fix it by checking if pagination is requested first.

The pull request was not tested. Please, consider it as an issue report with proposed solution.

P.S.:

Here is my log before

2018-07-27 10:53:09,741 DEBUG django.db.backends (0.350) SELECT COUNT(*) FROM (SELECT "core_agent"."id" AS Col1, (COALESCE(EXTRACT(EPOCH FROM now() - last_online_dt) < 30, False)) AS "is_online_db" FROM "core_agent" WHERE ("core_agent"."user_id" = 14 AND "core_agent"."is_hidden" = false AND "core_agent"."id" IN (SELECT DISTINCT U0."source_id" AS Col1 FROM "core_ping" U0 INNER JOIN "core_target" U1 ON (U0."target_id" = U1."id") WHERE (U0."ts" >= 1468962000 AND U0."ts" <= 1532638799 AND U1."name" = 'mikrotik.com'))) GROUP BY "core_agent"."id", (COALESCE(EXTRACT(EPOCH FROM now() - last_online_dt) < 30, False))) subquery; args=(30, 14, False, 1468962000, 1532638799, u'mikrotik.com', 30)
2018-07-27 10:53:10,063 DEBUG django.db.backends (0.320) SELECT "core_agent"."id", "core_agent"."user_id", "core_agent"."name", "core_agent"."key", "core_agent"."location", "core_agent"."model", "core_agent"."ip_address", "core_agent"."public_ip_address", "core_agent"."version", "core_agent"."last_heard_from", "core_agent"."last_online_dt", "core_agent"."notified_online_dt", "core_agent"."notified_offline_dt", "core_agent"."gps_longitude", "core_agent"."gps_latitude", "core_agent"."module_settings", "core_agent"."tags", "core_agent"."is_active", "core_agent"."is_hidden", "core_agent"."bytes_sent", "core_agent"."bytes_received", (COALESCE(EXTRACT(EPOCH FROM now() - last_online_dt) < 30, False)) AS "is_online_db" FROM "core_agent" WHERE ("core_agent"."user_id" = 14 AND "core_agent"."is_hidden" = false AND "core_agent"."id" IN (SELECT DISTINCT U0."source_id" AS Col1 FROM "core_ping" U0 INNER JOIN "core_target" U1 ON (U0."target_id" = U1."id") WHERE (U0."ts" >= 1468962000 AND U0."ts" <= 1532638799 AND U1."name" = 'mikrotik.com'))) ORDER BY "core_agent"."id" ASC; args=(30, 14, False, 1468962000, 1532638799, u'mikrotik.com')

And after the change:

2018-07-27 11:08:45,964 DEBUG django.db.backends (0.477) SELECT "core_agent"."id", "core_agent"."user_id", "core_agent"."name", "core_agent"."key", "core_agent"."location", "core_agent"."model", "core_agent"."ip_address", "core_agent"."public_ip_address", "core_agent"."version", "core_agent"."last_heard_from", "core_agent"."last_online_dt", "core_agent"."notified_online_dt", "core_agent"."notified_offline_dt", "core_agent"."gps_longitude", "core_agent"."gps_latitude", "core_agent"."module_settings", "core_agent"."tags", "core_agent"."is_active", "core_agent"."is_hidden", "core_agent"."bytes_sent", "core_agent"."bytes_received", (COALESCE(EXTRACT(EPOCH FROM now() - last_online_dt) < 30, False)) AS "is_online_db" FROM "core_agent" WHERE ("core_agent"."user_id" = 14 AND "core_agent"."is_hidden" = false AND "core_agent"."id" IN (SELECT DISTINCT U0."source_id" AS Col1 FROM "core_ping" U0 INNER JOIN "core_target" U1 ON (U0."target_id" = U1."id") WHERE (U0."ts" >= 1468962000 AND U0."ts" <= 1532638799 AND U1."name" = 'mikrotik.com'))) ORDER BY "core_agent"."id" ASC; args=(30, 14, False, 1468962000, 1532638799, u'mikrotik.com')

[codecov-io]

Codecov Report

Merging #6098 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #6098   +/-   ##
=======================================
  Coverage   96.17%   96.17%           
=======================================
  Files         128      128           
  Lines       17599    17599           
  Branches     1459     1459           
=======================================
  Hits        16926    16926           
  Misses        465      465           
  Partials      208      208

[tomchristie]

This one looks sufficiently self-evident that I'm happy with it, yup.