Re: [HACKERS] generic LONG VARLENA

Andreas Zeugswetter wrote:

> > I am excited about the long data type. This is _the_ way to do long
> > data types. Have any of the commercial databases figured out this way
> > to do it. I can't imagine a better system.
>
> The commercial db's usually make the dba decide on a per column basis
> whether the value is stored inside the table or in an extra space
> (blobspace,lobspace ...). They all have propietary syntax for this.
> (I would probably like it configurabe, whith some reasonable default)
> It is usually available for the text/byte and user defined datatypes.

Must have been proprietary syntax. There are may places in
SQL92 and SQL3 specs, where the words IMPLEMENTATION DEFINED
appear. With so many possible differences between
implementations within standard compliance, there must be
differences in the language too.

For the database schema, we cannot avoid proprietary syntax
to use implementation specific features. We don't have
tablespaces, extents etc., but if we ever implement something
like that, should we be unable to customize it because there
is no syntax defined in the standard?

> In PostgreSQL the array types come to mind.

There was a user request about "tuple too big" right today
when storing a polygon.

> What I think would be good is, if you could avoid the need for an index on
> the _LARGE_.. table.
> My Idea would be to store an xtid of the first lob page slot in the user
> table,
> and have an xtid pointer to the next lob page slot in it, and so on.
> That way you could avoid indices on the LARGE table.
> SnapshotAny() would also see the correct long, since an updated value would
> get a new xtid anyway. No need to use up an extra oid.

While I would like such an approach too, I don't want to do
it really. It would require to treat the lob tuples
different from regular ones in vacuum. It is one of the most
important tools for a productional DB. One single broken
xtid chain due to an aborted vacuum will corrupt your
database. Better keep it working the same as for a regular
table.

> Since lob's are typically large, the large overhead would be especially
> painful, so a different relkind with another pagelayout seems adequate.

No, I think a single Oid index on a relation, where only
usually large tuples are stored is a very small overhead.

Jan

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