Re: multivariate statistics (v19)

On Thu, Feb 2, 2017 at 3:59 AM, Tomas Vondra
<tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> There's a subtle difference between pg_node_tree and the data types for
> statistics - pg_node_tree stores the value as a string (matching the
> nodeToString output), so the _in function is fairly simple. Of course,
> stringToNode() assumes safe input, which is why the input is disabled.
>
> OTOH the statistics are stored in an optimized binary format, allowing to
> use the value directly (without having to do expensive parsing etc).
>
> I was thinking that the easiest way to add support for _in would be to add a
> bunch of Nodes for the statistics, along with in/out functions, but keeping
> the internal binary representation. But that'll be tricky to do in a safe
> way - even if those nodes are coded in a very defensive ways, I'd bet
> there'll be ways to inject unsafe nodes.
>
> So I'm OK with not having the _in for now. If needed, it's possible to
> construct the statistics as a bytea using a bit of C code. That's at least
> obviously unsafe, as anything written in C, touching the memory.

Since these data types are already special-purpose, I don't really see
why it would be desirable to entangle them with the existing code for
serializing and deserializing Nodes. Whether or not it's absolutely
necessary for these types to have input functions, it seems at least
possible that it would be useful, and it becomes much less likely that
we can make that work if it's piggybacking on stringToNode().

--
Robert Haas
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