Add secondary indexes to partitioned audit_events

What does this MR do?

Related to #241267 (closed)

This MR was split from !47581 (merged), so we could verify the indexes before swapping the tables. That MR contains the relevant details/discussion about the index management.

Indexes are created concurrently per-partition, before creating on the parent table:

rails db:migrate:up VERSION=20201112215028
== 20201112215028 AddPartitionedAuditEventIndexes: migrating ==================
-- index_name_exists?(:audit_events_part_5fc467ac26, "analytics_index_audit_events_part_on_created_at_and_author_id")
   -> 0.0008s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_000000", [:created_at, :author_id], {:name=>"index_2177f0dd80", :algorithm=>:concurrently})
   -> 0.0011s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_000000", [:created_at, :author_id], {:name=>"index_2177f0dd80", :algorithm=>:concurrently})
   -> 0.0030s
-- execute("RESET ALL")
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202011", [:created_at, :author_id], {:name=>"index_82131632c7", :algorithm=>:concurrently})
   -> 0.0005s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202011", [:created_at, :author_id], {:name=>"index_82131632c7", :algorithm=>:concurrently})
   -> 0.0020s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202012", [:created_at, :author_id], {:name=>"index_e84582b5f5", :algorithm=>:concurrently})
   -> 0.0005s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202012", [:created_at, :author_id], {:name=>"index_e84582b5f5", :algorithm=>:concurrently})
   -> 0.0017s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202101", [:created_at, :author_id], {:name=>"index_309cb73874", :algorithm=>:concurrently})
   -> 0.0005s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202101", [:created_at, :author_id], {:name=>"index_309cb73874", :algorithm=>:concurrently})
   -> 0.0016s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202102", [:created_at, :author_id], {:name=>"index_028162f283", :algorithm=>:concurrently})
   -> 0.0005s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202102", [:created_at, :author_id], {:name=>"index_028162f283", :algorithm=>:concurrently})
   -> 0.0016s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202103", [:created_at, :author_id], {:name=>"index_0043962d30", :algorithm=>:concurrently})
   -> 0.0005s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202103", [:created_at, :author_id], {:name=>"index_0043962d30", :algorithm=>:concurrently})
   -> 0.0016s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202104", [:created_at, :author_id], {:name=>"index_d42f8dc702", :algorithm=>:concurrently})
   -> 0.0005s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202104", [:created_at, :author_id], {:name=>"index_d42f8dc702", :algorithm=>:concurrently})
   -> 0.0017s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202105", [:created_at, :author_id], {:name=>"index_65b6207993", :algorithm=>:concurrently})
   -> 0.0005s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202105", [:created_at, :author_id], {:name=>"index_65b6207993", :algorithm=>:concurrently})
   -> 0.0017s
-- add_index(:audit_events_part_5fc467ac26, [:created_at, :author_id], {:name=>"analytics_index_audit_events_part_on_created_at_and_author_id"})
   -> 0.0021s
-- index_name_exists?(:audit_events_part_5fc467ac26, "idx_audit_events_part_on_entity_id_desc_author_id_created_at")
   -> 0.0004s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_000000", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_a71c4828a0", :algorithm=>:concurrently})
   -> 0.0010s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_000000", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_a71c4828a0", :algorithm=>:concurrently})
   -> 0.0018s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202011", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_63f4366262", :algorithm=>:concurrently})
   -> 0.0009s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202011", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_63f4366262", :algorithm=>:concurrently})
   -> 0.0015s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202012", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_acbdaa58e3", :algorithm=>:concurrently})
   -> 0.0011s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202012", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_acbdaa58e3", :algorithm=>:concurrently})
   -> 0.0016s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202101", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_f4e81fb9cf", :algorithm=>:concurrently})
   -> 0.0009s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202101", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_f4e81fb9cf", :algorithm=>:concurrently})
   -> 0.0017s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202102", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_0b3605a395", :algorithm=>:concurrently})
   -> 0.0008s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202102", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_0b3605a395", :algorithm=>:concurrently})
   -> 0.0015s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202103", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_b070ce7dc9", :algorithm=>:concurrently})
   -> 0.0007s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202103", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_b070ce7dc9", :algorithm=>:concurrently})
   -> 0.0015s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202104", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_1f4dff0a6d", :algorithm=>:concurrently})
   -> 0.0007s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202104", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_1f4dff0a6d", :algorithm=>:concurrently})
   -> 0.0013s
-- transaction_open?()
   -> 0.0000s
-- index_exists?("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202105", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_d9a68fddcf", :algorithm=>:concurrently})
   -> 0.0007s
-- add_index("gitlab_partitions_dynamic.audit_events_part_5fc467ac26_202105", [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"index_d9a68fddcf", :algorithm=>:concurrently})
   -> 0.0014s
-- add_index(:audit_events_part_5fc467ac26, [:entity_id, :entity_type, :id, :author_id, :created_at], {:order=>{:id=>:desc}, :name=>"idx_audit_events_part_on_entity_id_desc_author_id_created_at"})
   -> 0.0016s
== 20201112215028 AddPartitionedAuditEventIndexes: migrated (0.0736s) =========

Conversely, indexes are dropped by simply removing the parent index, which automatically recurses to all partitions:

rails db:migrate:down VERSION=20201112215028
== 20201112215028 AddPartitionedAuditEventIndexes: reverting ==================
-- index_name_exists?(:audit_events_part_5fc467ac26, "idx_audit_events_part_on_entity_id_desc_author_id_created_at")
   -> 0.0007s
-- remove_index(:audit_events_part_5fc467ac26, {:name=>"idx_audit_events_part_on_entity_id_desc_author_id_created_at"})
   -> 0.0013s
-- index_name_exists?(:audit_events_part_5fc467ac26, "analytics_index_audit_events_part_on_created_at_and_author_id")
   -> 0.0007s
-- remove_index(:audit_events_part_5fc467ac26, {:name=>"analytics_index_audit_events_part_on_created_at_and_author_id"})
   -> 0.0008s
== 20201112215028 AddPartitionedAuditEventIndexes: reverted (0.0241s) =========

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