72 lines
2.3 KiB
Ruby
72 lines
2.3 KiB
Ruby
# frozen_string_literal: true
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module Gitlab
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module Database
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module Count
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# A tablesample count executes in two phases:
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# * Estimate table sizes based on reltuples.
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# * Based on the estimate:
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# * If the table is considered 'small', execute an exact relation count.
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# * Otherwise, count on a sample of the table using TABLESAMPLE.
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#
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# The size of the sample is chosen in a way that we always roughly scan
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# the same amount of rows (see TABLESAMPLE_ROW_TARGET).
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#
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# There are no guarantees with respect to the accuracy of the result or runtime.
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class TablesampleCountStrategy < ReltuplesCountStrategy
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EXACT_COUNT_THRESHOLD = 10_000
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TABLESAMPLE_ROW_TARGET = 10_000
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def count
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estimates = size_estimates(check_statistics: false)
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models.each_with_object({}) do |model, count_by_model|
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count = perform_count(model, estimates[model])
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count_by_model[model] = count if count
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end
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rescue *CONNECTION_ERRORS
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{}
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end
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private
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def perform_count(model, estimate)
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# If we estimate 0, we may not have statistics at all. Don't use them.
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return unless estimate && estimate > 0
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if estimate < EXACT_COUNT_THRESHOLD
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# The table is considered small, the assumption here is that
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# the exact count will be fast anyways.
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model.count
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else
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# The table is considered large, let's only count on a sample.
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tablesample_count(model, estimate)
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end
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end
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def where_clause(model)
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return unless sti_model?(model)
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"WHERE #{model.inheritance_column} = '#{model.name}'"
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end
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def tablesample_count(model, estimate)
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portion = (TABLESAMPLE_ROW_TARGET.to_f / estimate).round(4)
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inverse = 1 / portion
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query = <<~SQL
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SELECT (COUNT(*)*#{inverse})::integer AS count
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FROM #{model.table_name}
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TABLESAMPLE SYSTEM (#{portion * 100})
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REPEATABLE (0)
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#{where_clause(model)}
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SQL
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rows = model.connection.select_all(query)
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Integer(rows.first['count'])
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end
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end
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end
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end
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end
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