MDEV-35813 Unnecessary InnoDB log writes in INSERT…SELECT

ha_innobase::extra(): Conditionally avoid a log write that had been
added in commit e5b9dc1536 (MDEV-25910)
because it may be invoked as part of select_insert::prepare_eof()
and not only during DDL operations.

Reviewed by: Sergei Golubchik

sql/sql_table.cc

@@ -11135,7 +11135,7 @@ do_continue:;
 
   debug_crash_here("ddl_log_alter_after_copy");      // Use old table
   /*
-    We are new ready to use the new table. Update the state in the
+    We are now ready to use the new table. Update the state in the
     ddl log so that we recovery know that the new table is ready and
     in case of crash it should use the new one and log the query
     to the binary log.

storage/innobase/handler/ha_innodb.cc

@@ -15958,9 +15958,22 @@ ha_innobase::extra(
 		break;
 	case HA_EXTRA_END_ALTER_COPY:
 		trx = check_trx_exists(ha_thd());
+		if (!m_prebuilt->table->skip_alter_undo) {
+			/* This could be invoked inside INSERT...SELECT.
+			We do not want any extra log writes, because
+			they could cause a severe performance regression. */
+			break;
+		}
 		m_prebuilt->table->skip_alter_undo = 0;
 		if (!m_prebuilt->table->is_temporary()
 		    && !high_level_read_only) {
+			/* The extra log write is necessary for
+			ALTER TABLE...ALGORITHM=COPY, because
+			a normal transaction commit would be a no-op
+			because no undo log records were generated.
+			This log write will also be unnecessarily executed
+			during CREATE...SELECT, which is the other caller of
+			handler::extra(HA_EXTRA_BEGIN_ALTER_COPY). */
 			log_buffer_flush_to_disk();
 		}
 		break;