Re: HeapTupleHeaderData Layout description
От | benj.dev@laposte.net |
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Тема | Re: HeapTupleHeaderData Layout description |
Дата | |
Msg-id | e83c6378-f298-2c81-5276-53b8f1c1bc71@laposte.net обсуждение исходный текст |
Ответ на | Re: HeapTupleHeaderData Layout description (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: HeapTupleHeaderData Layout description
(Tom Lane <tgl@sss.pgh.pa.us>)
|
Список | pgsql-docs |
Le 22/01/2021 à 17:49, Tom Lane a écrit : > benj.dev@laposte.net writes: >> In https://www.postgresql.org/docs/13/storage-page-layout.html<br> >> we can find on § 68.6.1. Table Row Layout<br> >> the Table 68.4. HeapTupleHeaderData Layout<br> >> <br> >> When additionning Length of differents Fields taht are present we obtain atotal of<br> >> 4+4+4+4+6+2+2+1 = 27<br> > > I think you missed the point that t_cid overlays with t_xvac. > > regards, tom lane > Hi, Thank you for this point. It's more clear. So in fact the error (with number 27) was in the documentation for version before postgres 8.3. In version >=8.3, the overlays precision could be added into sentence "XID for VACUUM operation moving a row version" => "XID for VACUUM operation moving a row version (overlays with t_cid)" or Maybe show t_cid and t_xvac in the same cell into the array could be more clear Example : |===========|===============|=========|================================| |Field |Type |Length |Description | |===========|===============|=========|================================| |t_xmin |TransactionId |4 octets |insert XID stamp | |-----------|---------------|---------|--------------------------------| |t_xmax |TransactionId |4 octets |delete XID stamp | |-----------|---------------|---------|--------------------------------| |t_cid |CommandId | |insert and/or delete CID stamp | |t_xvac |TransactionId | |XID for VACUUM operation moving | | | | |a row version | | | |4 octets |(t_cid and t_xvax are overlayed)| |-----------|---------------|---------|--------------------------------| |t_ctid |ItemPointerData|6 octets |current TID of this or newer row| | | | |version | |-----------|---------------|---------|--------------------------------| |t_infomask2|uint16 |2 octets |number of attributes, plus | | | | |various flag bits | |-----------|---------------|---------|--------------------------------| |t_infomask |uint16 |2 octets |various flag bits | |-----------|---------------|---------|--------------------------------| |t_hoff |uint8 |1 octet |offset to user data | |===========|===============|=========|================================| Thanks, Regards
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