postgresql unnamed statement

　　在pg的日志中经常会看到unname statement。它通常指的是unnamed prepared statement，由支持wire protocol的客户端（使用libpq的库）发起。通常是调用PQprepare方法，如下：

PGresult *PQprepare(PGconn *conn,
                    const char *stmtName,
                    const char *query,
                    int nParams,
                    const Oid *paramTypes);

stmtName can be "" to create an unnamed statement, in which case any pre-existing unnamed statement is automatically replaced。所以每次新的事务开始时，都会判断有没有unamed stmt，有的话清理掉，它不保存在公共的事务表，所以任何时候只存在一个。如下：

static void
exec_simple_query(const char *query_string)
{
...

    /*
     * Zap any pre-existing unnamed statement.  (While not strictly necessary,
     * it seems best to define simple-Query mode as if it used the unnamed
     * statement and portal; this ensures we recover any storage used by prior
     * unnamed operations.)
     */
    drop_unnamed_stmt();
...
}

https://www.postgresql.org/docs/current/libpq-exec.html

https://www.pgcon.org/2014/schedule/attachments/330_postgres-for-the-wire.pdf

https://stackoverflow.com/questions/46810606/postgres-can-i-prepare-unnamed-statement-from-sql