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Modifying TOAST thresholds

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: Modifying TOAST thresholds
Date: 2007-03-28 17:14:24
Message-ID: 4398.1175102064@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
In another thread I wrote:
> ... One thing I was just thinking about is that it's silly to have
> the threshold constrained so strongly by a desire that tuples in toast
> tables not be toastable.  It would be trivial to tweak the heapam.c
> routines so that they simply don't invoke the toaster when relkind is
> 't', and then we could have independent choices of toast-tuple size and
> main-tuple size.  This would be particularly good because in the current
> scheme you can't modify toast-tuple size without an initdb, but if that
> were decoupled there'd be no reason not to allow changes in the
> main-tuple thresholds.

After thinking about this more I'm convinced that the above is a good
idea, eg in heap_insert change

    if (HeapTupleHasExternal(tup) || tup->t_len > TOAST_TUPLE_THRESHOLD)
        heaptup = toast_insert_or_update(relation, tup, NULL, use_wal);
    else
        heaptup = tup;

to

    if (relation->rd_rel->relkind == RELKIND_TOASTVALUE)
    {
        /* toast table entries should never be recursively toasted */
        Assert(!HeapTupleHasExternal(tup));
        heaptup = tup;
    }
    else if (HeapTupleHasExternal(tup) || tup->t_len > TOAST_TUPLE_THRESHOLD)
        heaptup = toast_insert_or_update(relation, tup, NULL, use_wal);
    else
        heaptup = tup;

I also think that we ought to add TOAST_MAX_CHUNK_SIZE to the set of
compiled-in parameters that are recorded in pg_control and checked for
compatibility at startup (like BLCKSZ) --- this will prevent anyone from
shooting themselves in the foot while experimenting.

Any objections?

            regards, tom lane

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