TOAST pointer size

Previous Topic Next Topic
 
classic Classic list List threaded Threaded
2 messages Options
Reply | Threaded
Open this post in threaded view
|

TOAST pointer size

apt.postgresql.org Repository Update
The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/13/storage-toast.html
Description:

Hey team,

It would be nice to add a break-down of why the TOAST Pointer size is 18
bytes

 quoting here

 ```A pointer datum representing an out-of-line on-disk TOASTed value
therefore needs to store the OID of the TOAST table in which to look and the
OID of the specific value (its chunk_id). For convenience, pointer datums
also store the logical datum size (original uncompressed data length) and
physical stored size (different if compression was applied). Allowing for
the varlena header bytes, the total size of an on-disk TOAST pointer datum
is therefore 18 bytes regardless of the actual size of the represented
value.
```

Assuming OID is a 4 bytes unsigned, So 4 bytes for the TOAST Table OID + 4
bytes for the Chunk Id (OID)  + how many bytes for the physical stored size+
how many bytes for the logical datum size + 4 bytes for the verlena header
bytes ? Searched every where but couldn't find an answer how that adds up to
18 bytes
Reply | Threaded
Open this post in threaded view
|

Re: TOAST pointer size

Tom Lane-2
PG Doc comments form <[hidden email]> writes:
> It would be nice to add a break-down of why the TOAST Pointer size is 18
> bytes

Use the source, Luke.  The useful pointer data is indeed 16 bytes:

https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/postgres.h;h=c48f47e930ad05d3ae2e24b0c0c85662cd058a7b;hb=HEAD#l67

but there's varlena tag overhead:

https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/postgres.h;h=c48f47e930ad05d3ae2e24b0c0c85662cd058a7b;hb=HEAD#l159

This doesn't seem like appropriate detail for the user docs.

                        regards, tom lane