HeapTupleHeaderData Layout description

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HeapTupleHeaderData Layout description

benj.dev
‌Hi,

In https://www.postgresql.org/docs/13/storage-page-layout.html
we can find on § 68.6.1. Table Row Layout
the Table 68.4. HeapTupleHeaderData Layout

When additionning Length of differents Fields taht are present we obtain  atotal of
4+4+4+4+6+2+2+1 = 27

What doesn't seem to fit with previous indication "There is a fixed-size header (occupying 23 bytes on most machines)"

Researching in old doc shows that in 8.2 the documentation
https://www.postgresql.org/docs/8.2/storage-page-layout.html
indicate 27 bytes (with the same detailled Table)

Number 23 appears from Postgres 8.3

In some thread like https://stackoverflow.com/questions/2966524/calculating-and-saving-space-in-postgresql/7431468
It's possible to read that
Overhead per tuple (row)
... And at least 24 bytes (23 + padding) for the tuple header ...

As there is several thread about this point I suppose there is an evolution between 8.2 and 8.3 with a missdocumentation for an evolution of detailled Table "HeapTupleHeaderData Layout"

IS it right ? And in this case what is the real detailled Table ?

Thanks for any explanations
Regards
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Re: HeapTupleHeaderData Layout description

Tom Lane-2
[hidden email] 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


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Re: HeapTupleHeaderData Layout description

benj.dev
Le 22/01/2021 à 17:49, Tom Lane a écrit :

> [hidden email] 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


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Re: HeapTupleHeaderData Layout description

Tom Lane-2
[hidden email] writes:
> Le 22/01/2021 à 17:49, Tom Lane a écrit :
>> I think you missed the point that t_cid overlays with t_xvac.

> So in fact the error (with number 27) was in the documentation for
> version before postgres 8.3.

No, the pre-8.3 docs are also correct, for their time.

                        regards, tom lane