SimpleQuery　→　pool_send_and_wait  →　send_simplequery_message

/*                                    
 * Process Query('Q') message                                    
 * Query messages include an SQL string.                                    
 */                                    
POOL_STATUS SimpleQuery(POOL_CONNECTION *frontend, 
                POOL_CONNECTION_POOL *backend, int len, char *contents){
    ……                                
    /* log query to log file if necessary */                                
    if (pool_config->log_statement){                                
        pool_log("statement: %s", contents);                            
    }else{                                
        pool_debug("statement2: %s", contents);                            
    }                                
    ……                                
    string = query_context->original_query; 
    if (!RAW_MODE){                                
        ……                            
        /*                            
         * Query is not commit/rollback                            
         */                            
        if (!commit){                            
            char *rewrite_query;                        
            ……                        
            /*                        
             * Optimization effort: If there's only one session, we do 
             * not need to wait for the master node's response, and 
             * could execute the query concurrently.                        
             */                        
            if (pool_config->num_init_children == 1){                        
                /* Send query to all DB nodes at once */                    
                status = pool_send_and_wait(query_context, 0, 0); 
                /*                    
                free_parser();                    
                */                    
                return status;                    
            }                     
                                    
            /* Send the query to master node */                        
            if (pool_send_and_wait(query_context, 1, MASTER_NODE_ID) 

!= POOL_CONTINUE) {                        
                free_parser();                    
                return POOL_END;                    
            }                        
        }                         
        /*                            
         * Send the query to other than master node.                            
         */                            
        if (pool_send_and_wait(query_context, -1, MASTER_NODE_ID) 

!= POOL_CONTINUE{

free_parser();                        
            return POOL_END;                        
        }                            
        ……                            
    }else{                                
        ……                            
    }                                
    return POOL_CONTINUE;                                
}                                
                                    

/* 
* Send simple query and wait for response
* send_type:
* -1: do not send this node_id
* 0: send to all nodes
* >0: send to this node_id
*/
POOL_STATUS pool_send_and_wait(POOL_QUERY_CONTEXT *query_context,
int send_type, int node_id)
{
  ……
  /* Send query */
  for (i=0;i<NUM_BACKENDS;i++){
     ……
     per_node_statement_log(backend, i, string);

    if ( send_simplequery_message(CONNECTION(backend, i),

           len, string, MAJOR(backend)) != POOL_CONTINUE) { 

          return POOL_END;
    }
  }

  /* Wait for response */
  for (i=0;i<NUM_BACKENDS;i++){
    ……
    if (wait_for_query_response(frontend, CONNECTION(backend, i),

          MAJOR(backend)) != POOL_CONTINUE){
           /* Cancel current transaction */
           CancelPacket cancel_packet;

          cancel_packet.protoVersion = htonl(PROTO_CANCEL);
          cancel_packet.pid = MASTER_CONNECTION(backend)->pid;
          cancel_packet.key= MASTER_CONNECTION(backend)->key;
          cancel_request(&cancel_packet);

          return POOL_END;
    }
    ……
  }
return POOL_CONTINUE;
}