PostgreSQL在何处处理 sql查询之二十

/*
*    mdopen() -- Open the specified relation.
*
* Note we only open the first segment, when there are multiple segments.
*
* If first segment is not present, either ereport or return NULL according
* to "behavior".  We treat EXTENSION_CREATE the same as EXTENSION_FAIL;
* EXTENSION_CREATE means it's OK to extend an existing relation, not to
* invent one out of whole cloth.
*/
static MdfdVec *
mdopen(SMgrRelation reln, ForkNumber forknum, ExtensionBehavior behavior)
{
MdfdVec    *mdfd;
char       *path;
File        fd;

/* No work if already open */
if (reln->md_fd[forknum])
return reln->md_fd[forknum];

path = relpath(reln->smgr_rnode, forknum);

fd = PathNameOpenFile(path, O_RDWR | PG_BINARY, 0600);

if (fd < 0)
{
/*
* During bootstrap, there are cases where a system relation will be
* accessed (by internal backend processes) before the bootstrap
* script nominally creates it.  Therefore, accept mdopen() as a
* substitute for mdcreate() in bootstrap mode only. (See mdcreate)
*/
if (IsBootstrapProcessingMode())
fd = PathNameOpenFile(path, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
0600);
if (fd < 0)
{
if (behavior == EXTENSION_RETURN_NULL &&
FILE_POSSIBLY_DELETED(errno))
{
pfree(path);
return NULL;
}
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not open file \"%s\": %m", path)));
}
}

pfree(path);

reln->md_fd[forknum] = mdfd = _fdvec_alloc();

mdfd->mdfd_vfd = fd;
mdfd->mdfd_segno = 0;
mdfd->mdfd_chain = NULL;
Assert(_mdnblocks(reln, forknum, mdfd) <= ((BlockNumber) RELSEG_SIZE));

return mdfd;
}

/* No work if already open */
if (reln->md_fd[forknum])
return reln->md_fd[forknum];