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# For large tables, PostgreSQL can take a long time to count rows due to MVCC.
# We can optimize this by using the reltuples count as described in https://wiki.postgresql.org/wiki/Slow_Counting.
module Gitlab
module Database
module Count
CONNECTION_ERRORS =
if defined?(PG)
[
ActionView::Template::Error,
ActiveRecord::StatementInvalid,
PG::Error
].freeze
else
[
ActionView::Template::Error,
ActiveRecord::StatementInvalid
].freeze
end
def self.approximate_count(model)
return model.count unless Gitlab::Database.postgresql?
execute_estimate_if_updated_recently(model) || model.count
end
def self.execute_estimate_if_updated_recently(model)
ActiveRecord::Base.connection.select_value(postgresql_estimate_query(model)).to_i if reltuples_updated_recently?(model)
rescue *CONNECTION_ERRORS
end
def self.reltuples_updated_recently?(model)
time = "to_timestamp(#{1.hour.ago.to_i})"
query = <<~SQL
SELECT 1 FROM pg_stat_user_tables WHERE relname = '#{model.table_name}' AND
(last_vacuum > #{time} OR last_autovacuum > #{time} OR last_analyze > #{time} OR last_autoanalyze > #{time})
SQL
ActiveRecord::Base.connection.select_all(query).count > 0
rescue *CONNECTION_ERRORS
false
end
def self.postgresql_estimate_query(model)
"SELECT reltuples::bigint AS estimate FROM pg_class where relname = '#{model.table_name}'"
end
end
end
end
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