Determining number of rows in blocks

Question

Hello
Assume 8k blocksize
How can I know, how many rows will fit into one block?
Cheers

Answer 1

Not really no. What's a row look like? NUMBERs? VARACHARs? DATE types? If VARCHARs, the amount of data actually being stored could vary hugely.
If you already have some data loaded in a table somewhere, you may try sampling existing data to give you some idea.
Try something like this:
MBOBAK@mstmfgpep> create table test_rows nologging as select * from dba_tables;
Table created.
MBOBAK@mstmfgpep> select count(*) from TEST_ROWS;
  COUNT(*)
      1589
1 row selected.
MBOBAK@mstmfgpep> l
  1* select dbms_rowid.ROWID_to_absolute_fno(rowid,'MBOBAK','TEST_ROWS'),dbms_rowid.ROWID_BLOCK_NUMBER(rowid),count(*) rows_in_block from test_rows group by dbms_rowid.ROWID_to_absolute_fno(rowid,'MBOBAK','TEST_ROWS'),dbms_rowid.ROWID_BLOCK_NUMBER(rowid)
MBOBAK@mstmfgpep> /
DBMS_ROWID.ROWID_TO_ABSOLUTE_FNO(ROWID,'MBOBAK','TEST_ROWS') DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID) ROWS_IN_BLOCK
                                                           6                                  212            31
                                                           6                                  216            32
                                                           6                                  218            31
                                                           6                                  221            31
                                                           6                                  222            32
                                                           6                                  224            31
                                                           6                                  255            31
                                                           6                                  206            32
                                                           6                                  213            32
                                                           6                                  220            31
                                                           6                                  223            31
                                                           6                                  225            32
                                                           6                                  227            31
                                                           6                                  239            30
                                                           6                                  245            31
                                                           6                                  208            32
                                                           6                                  211            32
                                                           6                                  240            30
                                                           6                                  243            31
                                                           6                                  247            33
                                                           6                                  253            34
DBMS_ROWID.ROWID_TO_ABSOLUTE_FNO(ROWID,'MBOBAK','TEST_ROWS') DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID) ROWS_IN_BLOCK
                                                           6                                  204            32
                                                           6                                  228            31
                                                           6                                  230            31
                                                           6                                  232            31
                                                           6                                  234            31
                                                           6                                  235            30
                                                           6                                  250            41
                                                           6                                  254            33
                                                           6                                  256            31
                                                           6                                  207            31
                                                           6                                  237            30
                                                           6                                  241            31
                                                           6                                  242            32
                                                           6                                  244            32
                                                           6                                  246            31
                                                           6                                  248            38
                                                           6                                  251            34
                                                           6                                  209            32
                                                           6                                  231            31
                                                           6                                  236            30
                                                           6                                  238            30
DBMS_ROWID.ROWID_TO_ABSOLUTE_FNO(ROWID,'MBOBAK','TEST_ROWS') DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID) ROWS_IN_BLOCK
                                                           6                                  210            32
                                                           6                                  214            32
                                                           6                                  215            32
                                                           6                                  219            31
                                                           6                                  226            31
                                                           6                                  205            32
                                                           6                                  229            31
                                                           6                                  252            34
50 rows selected.
MBOBAK@mstmfgpep> This will count the number of rows that you're actually getting in each block. Warning, on a large table, this could take a very long time to run and produce lots of output. But you should be able to use the technique to sample how many rows/block you see on various tables.
Hope that helps,
-Mark