PostgreSQL在何处处理 sql查询之二十一

/*
 *    mdnblocks() -- Get the number of blocks stored in a relation.
 *
 *        Important side effect: all active segments of the relation are opened
 *        and added to the mdfd_chain list.  If this routine has not been
 *        called, then only segments up to the last one actually touched
 *        are present in the chain.
 */
BlockNumber
mdnblocks(SMgrRelation reln, ForkNumber forknum)
{
    MdfdVec    *v = mdopen(reln, forknum, EXTENSION_FAIL);
    BlockNumber nblocks;
    BlockNumber segno = 0;

    /*
     * Skip through any segments that aren't the last one, to avoid redundant
     * seeks on them.  We have previously verified that these segments are
     * exactly RELSEG_SIZE long, and it's useless to recheck that each time.
     *
     * NOTE: this assumption could only be wrong if another backend has
     * truncated the relation.    We rely on higher code levels to handle that
     * scenario by closing and re-opening the md fd, which is handled via
     * relcache flush.    (Since the checkpointer doesn't participate in
     * relcache flush, it could have segment chain entries for inactive
     * segments; that's OK because the checkpointer never needs to compute
     * relation size.)
     */
    while (v->mdfd_chain != NULL)
    {
        segno++;
        v = v->mdfd_chain;
    }

    for (;;)
    {
        nblocks = _mdnblocks(reln, forknum, v);
if (nblocks > ((BlockNumber) RELSEG_SIZE))
elog(FATAL, "segment too big");

if (nblocks < ((BlockNumber) RELSEG_SIZE))

return (segno * ((BlockNumber) RELSEG_SIZE)) + nblocks;

        ...

        v = v->mdfd_chain;
    }
}