Re: HeapTupleHeaderData Layout description

Le 22/01/2021 à 17:49, Tom Lane a écrit :
> benj(dot)dev(at)laposte(dot)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&nbsp; 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