104 lines
3.5 KiB
Ruby
104 lines
3.5 KiB
Ruby
# frozen_string_literal: true
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module EachBatch
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extend ActiveSupport::Concern
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class_methods do
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# Iterates over the rows in a relation in batches, similar to Rails'
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# `in_batches` but in a more efficient way.
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#
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# Unlike `in_batches` provided by Rails this method does not support a
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# custom start/end range, nor does it provide support for the `load:`
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# keyword argument.
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#
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# This method will yield an ActiveRecord::Relation to the supplied block, or
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# return an Enumerator if no block is given.
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#
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# Example:
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#
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# User.each_batch do |relation|
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# relation.update_all(updated_at: Time.current)
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# end
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#
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# The supplied block is also passed an optional batch index:
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#
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# User.each_batch do |relation, index|
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# puts index # => 1, 2, 3, ...
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# end
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#
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# You can also specify an alternative column to use for ordering the rows:
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#
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# User.each_batch(column: :created_at) do |relation|
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# ...
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# end
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#
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# This will produce SQL queries along the lines of:
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#
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# User Load (0.7ms) SELECT "users"."id" FROM "users" WHERE ("users"."id" >= 41654) ORDER BY "users"."id" ASC LIMIT 1 OFFSET 1000
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# (0.7ms) SELECT COUNT(*) FROM "users" WHERE ("users"."id" >= 41654) AND ("users"."id" < 42687)
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#
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# of - The number of rows to retrieve per batch.
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# column - The column to use for ordering the batches.
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# order_hint - An optional column to append to the `ORDER BY id`
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# clause to help the query planner. PostgreSQL might perform badly
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# with a LIMIT 1 because the planner is guessing that scanning the
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# index in ID order will come across the desired row in less time
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# it will take the planner than using another index. The
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# order_hint does not affect the search results. For example,
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# `ORDER BY id ASC, updated_at ASC` means the same thing as `ORDER
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# BY id ASC`.
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def each_batch(of: 1000, column: primary_key, order: :asc, order_hint: nil)
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unless column
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raise ArgumentError,
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'the column: argument must be set to a column name to use for ordering rows'
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end
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start = except(:select)
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.select(column)
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.reorder(column => order)
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start = start.order(order_hint) if order_hint
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start = start.take
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return unless start
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start_id = start[column]
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arel_table = self.arel_table
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1.step do |index|
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start_cond = arel_table[column].gteq(start_id)
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start_cond = arel_table[column].lteq(start_id) if order == :desc
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stop = except(:select)
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.select(column)
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.where(start_cond)
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.reorder(column => order)
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stop = stop.order(order_hint) if order_hint
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stop = stop
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.offset(of)
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.limit(1)
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.take
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relation = where(start_cond)
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if stop
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stop_id = stop[column]
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start_id = stop_id
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stop_cond = arel_table[column].lt(stop_id)
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stop_cond = arel_table[column].gt(stop_id) if order == :desc
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relation = relation.where(stop_cond)
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end
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# Any ORDER BYs are useless for this relation and can lead to less
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# efficient UPDATE queries, hence we get rid of it.
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relation = relation.except(:order)
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# Using unscoped is necessary to prevent leaking the current scope used by
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# ActiveRecord to chain `each_batch` method.
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unscoped { yield relation, index }
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break unless stop
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end
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end
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end
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end
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