v.19.7Bug Fixes

Fixed Very Rare Data Race Condition That Could Happen When Executing a Query with UNION ALL Involving at Least Two Selects from System.columns, System.tables, System.parts, System.parts_tables or Tables of Merge Family and Performing ALTER of Columns of the Related Tables Concurrently

Official CH changelog text
Fixed very rare data race condition that could happen when executing a query with UNION ALL involving at least two SELECTs from system.columns, system.tables, system.parts, system.parts_tables or tables of Merge family and performing ALTER of columns of the related tables concurrently. #5189 (alexey-milovidov)
PreviousNext