Hi, 

 

So first I'm a pgsql hacker newbie and I've been reading up on the storage
structure: 

http://www.postgresql.org/docs/8.2/interactive/storage-page-layout.html

 

I'm trying to recover deleted records from a page file (postgresql 8.2) :
i.e. base/dbId/20132

 

I am able to successfully read all the header data I need (PageHeaderData,
ItemIdData , HeapTupleHeaderData) 

but I hit a wall when I try to start reading user data. 

 

This has helped:

http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/postgres.h?rev=1.
77;content-type=text%2Fplain

 

I've read and understood fairly well how varlena structures are stored
(plain, compressed, external/toast) but so far I can't seem to read a plain
inline value.

I think part of my problem is I haven't really understood what 'Then make
sure you have the right alignment' means. 

 

My approach currently is:

After reading HeapTupleHeaderData (23 bytes), I advance another 4 bytes
(hoff) and try to read a 32 bit integer (first attribute).

 

I am expecting to get an integer value 1 but I get 512 .

 

Am I doing this wrong? 

Could someone point me to the pgsql code pieces I should be looking at? 

 

If useful, this is the information I have before reading the 'user data':

 

object(PostgreSQL_HeapTupleHeaderData)#14 (7) {

  ["xmin"]=>

  string(5) "13824"

  ["xmax"]=>

  string(1) "0"

  ["cid"]=>

  string(1) "0"

  ["ctid"]=>

  object(PostgreSQL_ItemPointerData)#16 (2) {

    ["blockId"]=>

    string(1) "0"

    ["posId"]=>

    int(0)

  }

  ["infomask2"]=>

  int(0)

  ["infomask"]=>

  int(2)

  ["hoff"]=>

  int(4)

}

object(PostgreSQL_Attribute)#7 (6) {

  ["name"]=>

  string(7) "book_id"

  ["relid"]=>

  int(20132)

  ["len"]=>

  int(4)

  ["num"]=>

  int(1)

  ["ndims"]=>

  int(0)

  ["align"]=>

  string(1) "i"

}

array(1) {

  ["book_id"]=>

  int(512)

}

Responses

• Re: Reading deleted records - PageHeader v3 at 2010-02-06 06:20:53 from Tom Lane

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