From 0b5b6197dd69e3a96489636563a16fef808552f7 Mon Sep 17 00:00:00 2001 From: hongyunyan <649330952@qq.com> Date: Tue, 14 Jun 2022 12:11:52 +0800 Subject: [PATCH] add comment for SchemaBuilder --- dbms/src/TiDB/Schema/SchemaBuilder.cpp | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/dbms/src/TiDB/Schema/SchemaBuilder.cpp b/dbms/src/TiDB/Schema/SchemaBuilder.cpp index 99e540e6c95..4c7fba1f362 100644 --- a/dbms/src/TiDB/Schema/SchemaBuilder.cpp +++ b/dbms/src/TiDB/Schema/SchemaBuilder.cpp @@ -370,7 +370,8 @@ void SchemaBuilder::applyAlterPhysicalTable(DBInfoPtr db_inf const auto & schema_change = schema_changes[i]; /// Update column infos by applying schema change in this step. schema_change.second(orig_table_info); - /// Update schema version aggressively for the sake of correctness. + /// Update schema version aggressively for the sake of correctness（for read part). + /// In read action, we will use table_info.schema_version(storage_version) and TiDBSchemaSyncer.cur_version(global_version) to compare with query_version, to decide whether we can read under this query_version, or we need to make the schema newer. In our comparison logic, we must ensure storage_version <= gloabl_version. Thus, when the table need apply multi diffs here, we must update schema version aggressively to ensure storage_version <= gloabl_version. The more detail info you can refer the comments in DAGStorageInterpreter::getAndLockStorages. orig_table_info.schema_version = target_version; auto alter_lock = storage->lockForAlter(getThreadName()); storage->alterFromTiDB(