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