Re: PATCH: pageinspect / add page_checksum and bt_page_items(bytea)

Hi,

I had a look into this patch and would like to share some of my review
comments that requires author's attention.

1) The comment for page_checksum() needs to be corrected. It seems
like it has been copied from page_header and not edited it further.

/*
* page_header
*
* Allows inspection of page header fields of a raw page
*/

PG_FUNCTION_INFO_V1(page_checksum);

Datum
page_checksum(PG_FUNCTION_ARGS)

2) It seems like you have choosen wrong datatype for page_checksum. I
am getting negative checksum value when trying to run below query. I
think the return type for the SQL function page_checksum should be
'integer' instead of 'smallint'.

postgres=# SELECT page_checksum(get_raw_page('pg_class', 0), 100);
page_checksum
---------------
-19532
(1 row)

Above problem also exist in case of page_header. I am facing similar
problem with page_header as well.

postgres=# SELECT page_header(get_raw_page('pg_class', 0));
page_header
---------------------------------------------
(0/2891538,-28949,1,220,7216,8192,8192,4,0)
(1 row)

3) Any reason for not marking bt_page_items or page_checksum as PARALLEL_SAFE.

4) Error messages in new bt_page_items are not consistent with old
bt_page_items. For eg. if i pass meta page blocknumber as input to
bt_page_items the error message thrown by new and old bt_page_items
are different.

postgres=# SELECT * FROM bt_page_items('btree_index', 0) LIMIT 1;
ERROR: block 0 is a meta page

postgres=# SELECT * FROM bt_page_items(get_raw_page('btree_index', 0)) LIMIT 1;
ERROR: block is a meta page

postgres=# SELECT * FROM bt_page_items(get_raw_page('btree_index',
1024)) LIMIT 1;
ERROR: block number 1024 is out of range for relation "btree_index"

postgres=# SELECT * FROM bt_page_items('btree_index', 1024) LIMIT 1;
ERROR: block number out of range

5) Code duplication in bt_page_items() and bt_page_items_bytea() needs
to be handled.

--
With Regards,
Ashutosh Sharma
EnterpriseDB:http://www.enterprisedb.com

On Tue, Mar 7, 2017 at 9:39 PM, Peter Eisentraut
<peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:
> On 3/6/17 16:33, Tomas Vondra wrote:
>>> I think it would be better not to maintain so much duplicate code
>>> between bt_page_items(text, int) and bt_page_items(bytea). How about
>>> just redefining bt_page_items(text, int) as an SQL-language function
>>> calling bt_page_items(get_raw_page($1, $2))?
>>>
>>
>> Maybe. We can also probably share the code at the C level, so I'll look
>> into that.
>
> I think SQL would be easier, but either way some reduction in
> duplication would be good.
>
>>> For page_checksum(), the checksums are internally unsigned, so maybe
>>> return type int on the SQL level, so that there is no confusion about
>>> the sign?
>>>
>>
>> That ship already sailed, I'm afraid. We already have checksum in the
>> page_header() output, and it's defined as smallint there. So using int
>> here would be inconsistent.
>
> OK, no worries then.
>
> --
> Peter Eisentraut http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
>
>
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