Class: PgCanary::Rules::CountStarWithoutWhere

Inherits:
Base
  • Object
show all
Defined in:
lib/pg_canary/rules/definitions/count_star_without_where.rb

Overview

SELECT COUNT(*) without WHERE. Because of MVCC, PostgreSQL has no O(1) row count — this scans the whole table.

Instance Method Summary collapse

Methods inherited from Base

all, check, default_enabled, enabled?, #initialize, option, options, rule_name

Constructor Details

This class inherits a constructor from PgCanary::Rules::Base

Instance Method Details

#check ⇒ Object 



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# File 'lib/pg_canary/rules/definitions/count_star_without_where.rb', line 12

def check
  detections = []
  each_scope do |scope|
    stmt = scope.stmt
    next if stmt.where_clause || stmt.group_clause.any?
    next unless count_star?(stmt)

    tables = scope.tables
    next unless tables.length == 1

    table = tables.first
    next unless applicable_table?(table)

    detections << detection(
      table: table,
      message: "COUNT(*) without WHERE scans the whole #{table} table — PostgreSQL's MVCC " \
               "has no O(1) row count.",
      suggestion: <<~SUGGESTION.chomp
        If an approximation is acceptable, use the planner's estimate:
          SELECT reltuples::bigint FROM pg_class WHERE relname = '#{table}';
        For exact counts displayed frequently, maintain a counter cache.
      SUGGESTION
    )
  end
  detections
end