libpq
 PQexec("SELECT * FROM pg_class;")
  PQsendQuery()
   pqPutMsgStart('Q')
   pqPuts("SELECT * FROM pg_class;")
   pqPutMsgEnd() 
   -> "QSELECT * FROM pg_class;"

postgres
 PostgresMain('Q')
  exec_simple_query()

   pg_parse_query("SELECT * FROM pg_class;")
    raw_parser()
     scanner_init()
     parser_init("SELECT", "*", "FROM", "pg_class") 
     -> SelectStmt

   pg_analyze_and_rewrite(SelectStmt)
    parse_analyze
     transformStmt()
    pg_rewrite_query()
     QueryRewrite
     -> Query

   pg_plan_queries(Query)
    pg_plan_query()
     planner()
      standard_planner()
       subquery_planner()
        grouping_planner()
         query_planner()
         -> Plan

   PortalRun(Plan)
    PortalRunSelect()
     ExecutorRun()
      standard_ExecutorRun()
       ExecutePlan()
        ExecProcNode(T_SeqScan)
        ExecSelect()
         printtup()
          pq_beginmessage('D')
          pq_sendint(22)
          OutputFunctionCall()
          pq_sendcountedtext()
          pq_endmessage() 
          -> 247 rows

libpq
 PQexec()
  PQexecFinish()
   PQgetResult()
    parseInput()
     pqParseInput3('D')
      getAnotherTuple()
       pqGetInt()
        pqGetnchar()
         pqAddTuple()
         -> 247 rows