Oracle extents - How can I have so many with this small size?

Question

This is a different question than the one with the almost-identical title.

Again, this is more curiosity about how Oracle Database works than anything I need to change (yes I know multiple extents are fine, I read Tom Kyte too).

I have a table with the following characteristics:

Initial Ext: 1.44GB
Next Ext:    1MB
Num Extents: 26
Size:        4.18MB

In this case, I got the size by scanning 10% of the table rows. With an initial extent size so high, and the data so low, how did further extents get added? I am certain that the table did not become huge, then shrink drastically back down again.

Thank you.

Answer 1

The data you're getting from Toad appears to be incorrect or, at least, misleading. If you are using a locally managed tablespace with automatic extent allocation, Oracle will determine your initial and next extent sizes automatically. In 11.2, the first 16 extents are going to be 64k in size (for a total of 1 MB). The next 63 extents are going to be 1 MB in size. So if you have 26 extents, that implies that the table occupies 11 MB of space on disk. An initial extent of 1.44 GB makes no sense and a size of 4.18 MB seems rather low if you're saying it hasn't shrunk.

What does

SELECT tablespace_name,
       extent_id, 
       bytes/1024 kb
  FROM user_extents
 WHERE segment_name = <<name of table>>

show you? If you have 26 extents, you should see that extents 0-15 are 64 kb in size and extents 16-25 are 1 MB in size.

SELECT SUM(bytes)/1024/1024 size_in_mb
  FROM user_extents
 WHERE segment_name = <<name of table>>

will show you the total size of the table's extents on disk.

Answer 2

To give details for future reference, Toad appears to be getting its information as follows:

Size:        dba_segments.bytes
Initial Ext: dba_segments.initial_extent
Next Ext:    dba_segments.next_extent
Num Extents: dba_segments.extent

This is not the 10% sampling I mentioned, but I have seen examples where the same disconnect appears (too many extents for small size).

The size matches the sum of the extent sizes from dba_extents, and the number matches the number of records in dba_extents, but the initial and next extent sizes appear unrelated. I would like to know where these numbers come from, since according to Oracle they're accurate, and they do have an effect (when importing tables, for example, you can't import a table with a huge initial extent size if there is not enough space for it in the tablespace, even though according to dba_extents, it's not using that much space).