Re: Support for 8-byte TOAST values, round two

<div dir='auto'><div><br><div><br><div class="elided-text">On Sep 14, 2026 05:02, Michael Paquier &lt;michael(at)paquier(dot)xyz&gt; wrote:<br type="attribution"><blockquote style="margin:0 0 0 0.8ex;border-left:1px #ccc solid;padding-left:1ex"><p dir="ltr">On Sun, Sep 13, 2026 at 10:07:27PM +0545, Oleg Bartunov wrote:
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&gt; I did a quick check of out-of-tree users and found at least two
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&gt; examples. PostGIS has code like this in ToastCacheGetGeometry():
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postgis holds a direct reference to varatt_external in its
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libpgcommon/lwgeom_cache.c, which is wanted to feed data to
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ToastCacheArgument.&nbsp; As far as I can see, their change is not
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complicated, upgrade ToastCacheArgument.valueid to be an Oid8, then,
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instead of the direct use of varatt_external() switch to a
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toast_external_data combined with a call of toast_external_info_get()
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to make the switch transparent, so as the TOAST value, Oid or Oid8 can
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be retrieved.
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So it's not a complicated change, and as far as I can see it's mostly
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a matter of two PG_VERSION_NUM blocks plus a few tests.&nbsp; Using
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varatt_external in the code would lead to a compilation failure,
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because it's now renamed varatt_external_oid, one will not see a
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runtime failure.&nbsp; It's going to be hard to miss that something is
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wrong in this case.
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&gt; MobilityDB carries the same pattern. So an OID8 datum can now pass
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&gt; VARATT_IS_EXTERNAL_ONDISK() and existing code can then interpret it
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&gt; using the old OID pointer layout.
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EXTERNAL_ONDISK() means either of them.
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&gt; I noticed this because we have been experimenting with alternative
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&gt; physical representations of large varlena values and tracing which
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&gt; parts of the value lifecycle actually depend on a particular TOAST
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&gt; representation. OID8 turned out to be a useful real-world test of that
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&gt; boundary.
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MobilityDB is carrying the same patterns because it uses a copy-paste
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of the postgis code.&nbsp; Again, this uses directly varatt_external, which
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would fail at compilation under PG20.&nbsp; It sounds to me that they just
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update their code on a regular basis based on postgis, so it means
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that sending only a patch to adjust postgis would be enough.&nbsp; I'm
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happy to take some time to do that, it</p></blockquote></div></div></div><div dir="auto">Thanks, that makes sense. I had missed that the varatt_external rename turns this particular case into a compile-time failure.</div><div dir="auto"><div><div class="elided-text"><blockquote style="margin:0 0 0 0.8ex;border-left:1px #ccc solid;padding-left:1ex"><p dir="ltr"><br>
&gt; Perhaps we need to distinguish explicitly between "any on-disk
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&gt; external pointer" and "the old OID on-disk pointer", rather than
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&gt; changing the meaning of the existing predicate?
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It seems to me that this difference already exists in the patch, due
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to the rebranding of varatt_external.
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Note that if somebody has the idea to use VARTAG_SIZE() to copy
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external varlena pointers, then the change is transparent.</p></blockquote></div></div></div><div dir="auto"><br></div><div dir="auto">Just to make sure I understand the intended abstraction correctly: with this patch, should I think of VARATT_IS_EXTERNAL_ONDISK() as identifying a family of on-disk external representations, while the vartag identifies the concrete physical representation within that family?</div><div dir="auto">In other words:</div><div dir="auto">VARATT_IS_EXTERNAL_ONDISK() -&gt; representation family, and&nbsp;</div><div dir="auto">VARTAG_* -&gt; concrete representation</div><div dir="auto">If so, that is actually the distinction I was looking for.</div><div dir="auto"><br></div><div dir="auto">Oleg&nbsp;</div><div dir="auto"><div><div class="elided-text"><blockquote style="margin:0 0 0 0.8ex;border-left:1px #ccc solid;padding-left:1ex"><p dir="ltr">
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--
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Michael
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</p>
</blockquote></div><br></div></div></div>