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);fprintf(stderr,"%d blocks by process %d\n\n",nblocks,getpid());if (nblocks > ((BlockNumber) RELSEG_SIZE))elog(FATAL, "segment too big");if (nblocks < ((BlockNumber) RELSEG_SIZE))return (segno * ((BlockNumber) RELSEG_SIZE)) + nblocks;/** If segment is exactly RELSEG_SIZE, advance to next one.*/segno++;if (v->mdfd_chain == NULL){/** Because we pass O_CREAT, we will create the next segment (with* zero length) immediately, if the last segment is of length* RELSEG_SIZE.  While perhaps not strictly necessary, this keeps* the logic simple.*/v->mdfd_chain = _mdfd_openseg(reln, forknum, segno, O_CREAT);if (v->mdfd_chain == NULL)ereport(ERROR,(errcode_for_file_access(),errmsg("could not open file \"%s\": %m",_mdfd_segpath(reln, forknum, segno))));}v = v->mdfd_chain;}
}

postgres=# create table tst01(id integer);
CREATE TABLE
postgres=# postgres=# select oid from pg_class where relname='tst01';oid
-------16384
(1 row)

[root@lex base]# ls ./12788/16384
./12788/16384postgres=# insert into tst01 values(generate_series(1,2048));
INSERT 0 2048
postgres=# [root@lex base]# ls -lrt ./12788/16384
-rw------- 1 postgres postgres 81920 May 28 11:54 ./12788/16384
[root@lex base]# ls -lrt -kb ./12788/16384
-rw------- 1 postgres postgres 80 May 28 11:54 ./12788/16384
[root@lex base]# 

postgres=# insert into tst01 values(generate_series(2049,4096));
INSERT 0 2048
postgres=#[root@lex base]# ls -lrt -kb ./12788/16384
-rw------- 1 postgres postgres 152 May 28 11:56 ./12788/16384
[root@lex base]# 

postgres=# select pg_column_size(id) from tst01 limit 1;pg_column_size
----------------4
(1 row)postgres=# select pg_column_size(t) from tst01 t limit 1;pg_column_size
----------------28
(1 row)

postgres=# select count(*) from tst01;count
-------4096
(1 row)postgres=# 

19 blocks by process 4920

[root@lex 12788]# ls -lrt 16384
-rw------- 1 postgres postgres 155648 May 28 11:58 16384
[root@lex 12788]# 155648/8096 = 19.225296442688