# UNDF: UNDF-2026-000000255
Fixes rails-0016: SQLite3Adapter#copy_table_indexes + copy_table_contents — to_column_names and from_columns are Arrays used inside .select and .find_all loops O(I×C×N).

--- a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ DEFECT rails-0016a: lines 705-730 copy_table_indexes

         def copy_table_indexes(from, to, rename = {})
           indexes(from).each do |index|
             ...
             columns = index.columns
             if columns.is_a?(Array)
-              to_column_names = columns(to).map(&:name)               # Array rebuilt per index
-              columns = columns.map { |c| rename[c] || c }.select do |column|
-                to_column_names.include?(column)                      # O(N) Array scan per column
+              to_column_names_set = columns(to).map(&:name).to_set    # FIX: build Set once per index
+              columns = columns.map { |c| rename[c] || c }.select do |column|
+                to_column_names_set.include?(column)                  # O(1) per column
               end
             end
             ...
           end
         end

@@ DEFECT rails-0016b: lines 733-737 copy_table_contents

         def copy_table_contents(from, to, columns, rename = {})
           column_mappings = Hash[columns.map { |name| [name, name] }]
           rename.each { |a| column_mappings[a.last] = a.first }
-          from_columns = columns(from).collect(&:name)               # Array
-          columns = columns.find_all { |col| from_columns.include?(column_mappings[col]) }  # O(C×N)
+          from_columns_set = columns(from).collect(&:name).to_set    # FIX: Set for O(1) lookup
+          columns = columns.find_all { |col| from_columns_set.include?(column_mappings[col]) }
           ...
         end

# BEFORE: copy_table_indexes: to_column_names is Array rebuilt for each index; include? O(N) per column
#         I indexes × C columns per index × N columns per table = O(I×C×N)
#         copy_table_contents: from_columns is Array; find_all with include? is O(C×N)
# AFTER:  to_column_names_set / from_columns_set are Sets: O(1) include?
#         copy_table_indexes: O(I×C); copy_table_contents: O(C)
# Severity: MEDIUM — hot during ALTER TABLE simulations (SQLite has no native ALTER)
#           large tables with many indexes trigger this repeatedly
# Speedup: ~N× where N = column count
# At I=20 indexes, C=10 col/index, N=50 columns: 10,000 → 200 ops