Harden CI pipelines usage data queries

What does this MR do?

Adds a better index index_ci_pipelines_on_user_id_and_created_at_and_source to cover ci_builds.source conditions which helps the below counters

• We precompute the MIN/MAX values for the user_id as start/finish parameters
• For Query 1 Count query
  • Before https://explain.depesz.com/s/tsde ~203 seconds
  • After https://explain.depesz.com/s/b8Qb ~120 milliseconds
• We see the same improvement in all 4 counters where optimizer chooses source filter or not depending on the data
• Drops previous index index_ci_pipelines_on_user_id_and_created_at !26774 (merged)
• The new index is backwards compatible in order of columns

Read main issue #220477 (closed) for investigation

Optimization

DROP INDEX index_ci_pipelines_on_user_id_and_created_at
CREATE INDEX index_ci_pipelines_on_user_id_and_created_at_and_source ON public.ci_pipelines USING btree (user_id, created_at, source)

Ruby code and SQLs

Query 1

time_period = {}

ci_external_pipelines: Gitlab::UsageData.distinct_count(::Ci::Pipeline.external.where(time_period), :user_id),
SELECT COUNT(DISTINCT "ci_pipelines"."user_id") FROM "ci_pipelines" WHERE "ci_pipelines"."source" = 6 AND "ci_pipelines"."user_id" BETWEEN 0 AND 1250

• Before https://explain.depesz.com/s/u6DRY
• After https://explain.depesz.com/s/FE9H

Query 2

ci_internal_pipelines: Gitlab::UsageData.distinct_count(::Ci::Pipeline.internal.where(time_period), :user_id)
SELECT COUNT(DISTINCT "ci_pipelines"."user_id") FROM "ci_pipelines" WHERE ("ci_pipelines"."source" IN (1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12) OR "ci_pipelines"."source" IS NULL) AND "ci_pipelines"."user_id" BETWEEN 1 AND 1250

• Before https://explain.depesz.com/s/Xge1
• After https://explain.depesz.com/s/420T

Index size & timing

• Drop index takes 23 seconds
• Create index takes 20 minutes

gitlabhq_production=> \di+  index_ci_pipelines_on_user_id_and_created_at
                                               List of relations
 Schema |                     Name                     | Type  | Owner  |    Table     |  Size   | Description 
--------+----------------------------------------------+-------+--------+--------------+---------+-------------
 public | index_ci_pipelines_on_user_id_and_created_at | index | gitlab | ci_pipelines | 6285 MB | 
(1 row)

\di+ index_ci_pipelines_on_user_id_and_created_at_and_source

 Schema |                          Name                           | Type  | Owner  |    Table     |  Size   | Description   
--------+---------------------------------------------------------+-------+--------+--------------+---------+-------------  
 public | index_ci_pipelines_on_user_id_and_created_at_and_source | index | gitlab | ci_pipelines | 5534 MB |   
(1 row)  

Migration output

 VERBOSE=true bundle exec rake db:migrate:up VERSION=20200608075553
== 20200608075553 AddIndexOnUserIdAndCreatedAtAndSourceToCiPipelines: migrating 
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:ci_pipelines, [:user_id, :created_at, :source], {:algorithm=>:concurrently})
   -> 0.0059s
-- add_index(:ci_pipelines, [:user_id, :created_at, :source], {:algorithm=>:concurrently})
   -> 0.0242s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:ci_pipelines, [:user_id, :created_at], {:algorithm=>:concurrently})
   -> 0.0197s
-- remove_index(:ci_pipelines, {:algorithm=>:concurrently, :column=>[:user_id, :created_at]})
   -> 0.0244s
== 20200608075553 AddIndexOnUserIdAndCreatedAtAndSourceToCiPipelines: migrated (0.0756s) 

aakgun@saygitu:~/aakgun/1/gdk/gitlab$ VERBOSE=true bundle exec rake db:migrate:down VERSION=20200608075553
== 20200608075553 AddIndexOnUserIdAndCreatedAtAndSourceToCiPipelines: reverting 
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:ci_pipelines, [:user_id, :created_at], {:algorithm=>:concurrently})
   -> 0.0056s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:ci_pipelines, [:user_id, :created_at, :source], {:algorithm=>:concurrently})
   -> 0.0036s
-- remove_index(:ci_pipelines, {:algorithm=>:concurrently, :column=>[:user_id, :created_at, :source]})
   -> 0.0105s
== 20200608075553 AddIndexOnUserIdAndCreatedAtAndSourceToCiPipelines: reverted (0.0202s) 

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team