Re: Add visibility map information to pg_freespace.

Hello, 

> I'm looking into this patch as a reviewer.

I'd appreciate your time to review.


I've had some suggestions so far,
- I should be cautious in changing existing interface.
 You're right. It was somehow gone out of my mind. It might be better to provide a separate function from the
compatibilityview despite the loss of the pertinence to stay in this extension. However, it is too small to be a
standaloneextension.
 
 On the other hand the newly-added-column-to-the-tail could be said to be harmless for the most cases considering the
usageof this extension, I suppose.
 

- Historical note is needed in pg_freespace doc.
 Agreed, I'll provide documents not only for freespace, but for other modules I'll touch in this patch later.

- How about pageinspect?
 I proposed a simple representation format as a basis for discussion. Nevertheless, the VM pages has no more structure
thana simple bit string. Given the VM info in pg_freespacemap, I've come in doubt of the necessity of
vm_page_contnets()for the reason besides the orthogonality in the this extension's interface (which paid no attention
before:-).

- How about pgstattuple?
 It could even be said to be meaningful to add the number of not-all-visible pages or the ratio of it in the total
pages..
  | postgres=# select * from pgstattuple('t');  | -[ RECORD 1 ]----------------+---------  | table_len
 | 88711168  | tuple_count                  | 600001  | tuple_len                    | 26400044  | tuple_percent
       | 29.76  | dead_tuple_count             | 399999  | dead_tuple_len               | 17599956  |
dead_tuple_percent          | 19.84  | free_space                   | 33607960  | free_percent                 | 37.88
+not_all_visible_page_percent | 23.54
 

# This column name looks too long, though.
 In addition, the discussion above about the stability of the interface is also applicable to this.


Any suggestions?

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center