68 lines
2.2 KiB
Ruby
68 lines
2.2 KiB
Ruby
|
# frozen_string_literal: true
|
||
|
|
|
||
|
|
module LooseIndexScan
|
||
|
|
extend ActiveSupport::Concern
|
||
|
|
|
||
|
|
class_methods do
|
||
|
|
# Builds a recursive query to read distinct values from a column.
|
||
|
|
#
|
||
|
|
# Example 1: collect all distinct author ids for the `issues` table
|
||
|
|
#
|
||
|
|
# Bad: The DB reads all issues, sorts and dedups them in memory
|
||
|
|
#
|
||
|
|
# > Issue.select(:author_id).distinct.map(&:author_id)
|
||
|
|
#
|
||
|
|
# Good: Use loose index scan (skip index scan)
|
||
|
|
#
|
||
|
|
# > Issue.loose_index_scan(column: :author_id).map(&:author_id)
|
||
|
|
#
|
||
|
|
# Example 2: List of users for the DONE todos selector. Select all users who created a todo.
|
||
|
|
#
|
||
|
|
# Bad: Loads all DONE todos for the given user and extracts the author_ids
|
||
|
|
#
|
||
|
|
# > User.where(id: Todo.where(user_id: 4156052).done.select(:author_id))
|
||
|
|
#
|
||
|
|
# Good: Loads distinct author_ids from todos and then loads users
|
||
|
|
#
|
||
|
|
# > distinct_authors = Todo.where(user_id: 4156052).done.loose_index_scan(column: :author_id).select(:author_id)
|
||
|
|
# > User.where(id: distinct_authors)
|
||
|
|
def loose_index_scan(column:, order: :asc)
|
||
|
|
arel_table = self.arel_table
|
||
|
|
arel_column = arel_table[column.to_s]
|
||
|
|
|
||
|
|
cte = Gitlab::SQL::RecursiveCTE.new(:loose_index_scan_cte, union_args: { remove_order: false })
|
||
|
|
|
||
|
|
cte_query = except(:select)
|
||
|
|
.select(column)
|
||
|
|
.order(column => order)
|
||
|
|
.limit(1)
|
||
|
|
|
||
|
|
inner_query = except(:select)
|
||
|
|
|
||
|
|
cte_query, inner_query = yield([cte_query, inner_query]) if block_given?
|
||
|
|
cte << cte_query
|
||
|
|
|
||
|
|
inner_query = if order == :asc
|
||
|
|
inner_query.where(arel_column.gt(cte.table[column.to_s]))
|
||
|
|
else
|
||
|
|
inner_query.where(arel_column.lt(cte.table[column.to_s]))
|
||
|
|
end
|
||
|
|
|
||
|
|
inner_query = inner_query.order(column => order)
|
||
|
|
.select(column)
|
||
|
|
.limit(1)
|
||
|
|
|
||
|
|
cte << cte.table
|
||
|
|
.project(Arel::Nodes::Grouping.new(Arel.sql(inner_query.to_sql)).as(column.to_s))
|
||
|
|
|
||
|
|
unscoped do
|
||
|
|
select(column)
|
||
|
|
.with
|
||
|
|
.recursive(cte.to_arel)
|
||
|
|
.from(cte.alias_to(arel_table))
|
||
|
|
.where(arel_column.not_eq(nil)) # filtering out the last NULL value
|
||
|
|
end
|
||
|
|
end
|
||
|
|
end
|
||
|
|
end
|