Optimization: in the replication slave, we can avoid doing one strlen() per event's execution,

as we already have db_len in Log_event. Only if rewrite_db() changed the db we need a strlen
(so we now do the strlen() in rewrite_db). Plus a test (we had none for --replicate-rewrite-db :( ).


sql/log_event.cc:
  The goal is to get of rid of one strlen() per replication event in slave:
  we don't need to compute strlen(thd->db) as we already have db_len in the event;
  only case where we need to do a strlen() is if rewrite_db() changed the db.
  Note that db_len is always a meaningful value. It's 0 if event's db is 0.
sql/slave.cc:
  rewrite_db now returns the len of the returned db.
  print_slave_db_safe() needn't call rewrite_db() as rewrite_db() is already called by caller.
sql/slave.h:
  declaration updates for slave.cc

mysql-test/r/rpl_rewrite_db.result

+stop slave;
+drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
+reset master;
+reset slave;
+drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
+start slave;
+drop database if exists mysqltest1;
+create database mysqltest1;
+use mysqltest1;
+create table t1 (a int);
+insert into t1 values(9);
+select * from mysqltest1.t1;
+a
+9
+show databases like 'mysqltest1';
+Database (mysqltest1)
+mysqltest1
+select * from test.t1;
+a
+9
+drop table t1;
+drop database mysqltest1;

mysql-test/t/rpl_rewrite_db-slave.opt

+"--replicate-rewrite-db=mysqltest1->test"

mysql-test/t/rpl_rewrite_db.test

+source include/master-slave.inc;
+--disable_warnings
+drop database if exists mysqltest1;
+--enable_warnings
+create database mysqltest1;
+
+use mysqltest1;
+create table t1 (a int);
+insert into t1 values(9);
+select * from mysqltest1.t1;
+sync_slave_with_master;
+show databases like 'mysqltest1'; # should be empty
+select * from test.t1;
+# cleanup
+connection master;
+drop table t1;
+drop database mysqltest1;
+sync_slave_with_master;
+

sql/log_event.cc

@@ -977,7 +977,8 @@ void Query_log_event::print(FILE* file, bool short_form, char* last_db)
 int Query_log_event::exec_event(struct st_relay_log_info* rli)
 {
   int expected_error,actual_error= 0;
-  thd->db= (char*) rewrite_db(db); // thd->db_length is set later if needed
+  thd->db_length= db_len;
+  thd->db= (char*) rewrite_db(db, &thd->db_length);
 
   /*
     InnoDB internally stores the master log position it has processed so far;
@@ -995,11 +996,6 @@ int Query_log_event::exec_event(struct st_relay_log_info* rli)
   if (db_ok(thd->db, replicate_do_db, replicate_ignore_db))
   {
     thd->set_time((time_t)when);
-    /*
-      We cannot use db_len from event to fill thd->db_length, because
-      rewrite_db() may have changed db.
-    */ 
-    thd->db_length= thd->db ? strlen(thd->db) : 0;
     thd->query_length= q_len;
     thd->query = (char*)query;
     VOID(pthread_mutex_lock(&LOCK_thread_count));
@@ -1057,7 +1053,7 @@ Default database: '%s'. Query: '%s'",
 			expected_error,
 			actual_error ? thd->net.last_error: "no error",
 			actual_error,
-			print_slave_db_safe(db), query);
+			print_slave_db_safe(thd->db), query);
       thd->query_error= 1;
     }
     /*
@@ -1078,7 +1074,7 @@ Default database: '%s'. Query: '%s'",
 			"Error '%s' on query. Default database: '%s'. Query: '%s'",
 			(actual_error ? thd->net.last_error :
 			 "unexpected success or fatal error"),
-			print_slave_db_safe(db), query);
+			print_slave_db_safe(thd->db), query);
       thd->query_error= 1;
     }
   } /* End of if (db_ok(... */
@@ -1706,7 +1702,8 @@ int Load_log_event::exec_event(NET* net, struct st_relay_log_info* rli,
 			       bool use_rli_only_for_errors)
 {
   char *load_data_query= 0;
-  thd->db= (char*) rewrite_db(db); // thd->db_length is set later if needed
+  thd->db_length= db_len;
+  thd->db= (char*) rewrite_db(db, &thd->db_length);
   DBUG_ASSERT(thd->query == 0);
   thd->query_length= 0;                         // Should not be needed
   thd->query_error= 0;
@@ -1741,7 +1738,6 @@ int Load_log_event::exec_event(NET* net, struct st_relay_log_info* rli,
   if (db_ok(thd->db, replicate_do_db, replicate_ignore_db))
   {
     thd->set_time((time_t)when);
-    thd->db_length= thd->db ? strlen(thd->db) : 0;
     VOID(pthread_mutex_lock(&LOCK_thread_count));
     thd->query_id = query_id++;
     VOID(pthread_mutex_unlock(&LOCK_thread_count));
@@ -1847,7 +1843,7 @@ Slave: load data infile on table '%s' at log position %s in log \
                         (char*) table_name,
                         llstr(log_pos,llbuff), RPL_LOG_NAME, 
 			(ulong) thd->cuted_fields,
-                        print_slave_db_safe(db));
+                        print_slave_db_safe(thd->db));
       }
       if (net)
         net->pkt_nr= thd->net.pkt_nr;
@@ -1865,6 +1861,7 @@ Slave: load data infile on table '%s' at log position %s in log \
   }
 	    
   thd->net.vio = 0; 
+  char *save_db= thd->db;
   VOID(pthread_mutex_lock(&LOCK_thread_count));
   thd->db= 0;
   thd->query= 0;
@@ -1887,7 +1884,7 @@ Slave: load data infile on table '%s' at log position %s in log \
     }
     slave_print_error(rli,sql_errno,"\
 Error '%s' running LOAD DATA INFILE on table '%s'. Default database: '%s'",
-		      err, (char*)table_name, print_slave_db_safe(db));
+		      err, (char*)table_name, print_slave_db_safe(save_db));
     free_root(&thd->mem_root,MYF(MY_KEEP_PREALLOC));
     return 1;
   }
@@ -1897,7 +1894,7 @@ Error '%s' running LOAD DATA INFILE on table '%s'. Default database: '%s'",
   {
     slave_print_error(rli,ER_UNKNOWN_ERROR, "\
 Fatal error running LOAD DATA INFILE on table '%s'. Default database: '%s'",
-		      (char*)table_name, print_slave_db_safe(db));
+		      (char*)table_name, print_slave_db_safe(save_db));
     return 1;
   }
 

sql/slave.cc

@@ -1032,7 +1032,7 @@ bool net_request_file(NET* net, const char* fname)
 }
 
 
-const char *rewrite_db(const char* db)
+const char *rewrite_db(const char* db, uint *new_len)
 {
   if (replicate_rewrite_db.is_empty() || !db)
     return db;
@@ -1042,8 +1042,11 @@ const char *rewrite_db(const char* db)
   while ((tmp=it++))
   {
     if (!strcmp(tmp->key, db))
+    {
+      *new_len= strlen(tmp->val);
       return tmp->val;
     }
+  }
   return db;
 }
 
@@ -1056,7 +1059,7 @@ const char *rewrite_db(const char* db)
 
 const char *print_slave_db_safe(const char* db)
 {
-  return (db ? rewrite_db(db) : "");
+  return (db ? db : "");
 }
 
 /*

sql/slave.h

@@ -510,8 +510,8 @@ int add_table_rule(HASH* h, const char* table_spec);
 int add_wild_table_rule(DYNAMIC_ARRAY* a, const char* table_spec);
 void init_table_rule_hash(HASH* h, bool* h_inited);
 void init_table_rule_array(DYNAMIC_ARRAY* a, bool* a_inited);
-const char *rewrite_db(const char* db);
-const char *print_slave_db_safe(const char* db);
+const char *rewrite_db(const char* db, uint *new_db_len);
+const char *print_slave_db_safe(const char *db);
 int check_expected_error(THD* thd, RELAY_LOG_INFO* rli, int error_code);
 void skip_load_data_infile(NET* net);
 void slave_print_error(RELAY_LOG_INFO* rli, int err_code, const char* msg, ...);