54 lines
1.5 KiB
Ruby
54 lines
1.5 KiB
Ruby
# frozen_string_literal: true
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# Parse the output of the Postgres EXPLAIN command to get how many rows Postgres
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# *thinks* a SQL query will return. This is an estimate, and only accurate for
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# queries that have a single condition. If the query has multiple conditions,
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# then Postgres will assume they're independent (have no overlap), which is not
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# always the case.
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#
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# Used by {PostQueryBuilder#fast_count} to get a fast post count estimate for
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# certain single metatag searches.
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#
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# ExplainParser.new(Post.all).query_plan
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# => EXPLAIN (FORMAT JSON) SELECT "posts".* FROM "posts"
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# => {
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# "Node Type"=>"Seq Scan",
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# "Parallel Aware"=>false,
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# "Relation Name"=>"posts",
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# "Alias"=>"posts",
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# "Startup Cost"=>0.0,
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# "Total Cost"=>780900.02,
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# "Plan Rows"=>4413102,
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# "Plan Width"=>1268
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# }
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#
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# @see https://www.postgresql.org/docs/current/sql-explain.html
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class ExplainParser
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extend Memoist
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attr_reader :relation
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# @param the ActiveRecord relation
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def initialize(relation)
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@relation = relation
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end
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# @return [Hash] the Postgres query plan
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def query_plan
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result = ApplicationRecord.connection.select_one("EXPLAIN (FORMAT JSON) #{sql}")
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json = JSON.parse(result["QUERY PLAN"])
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json.first["Plan"]
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end
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# @return [Integer] the number of rows Postgres *thinks* the query will return
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def row_count
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query_plan["Plan Rows"]
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end
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# @return [String] the query's SQL
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def sql
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relation.reorder(nil).to_sql
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end
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memoize :query_plan
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end
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