Re: machine-readable explain output

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Jun 16, 2009 at 12:04 PM, Tom Lane<tgl@sss.pgh.pa.us> wrote:
>> FWIW, I like Greg's idea of subdividing the available data this way.

> I like it too, but I'd like to see us come up with a design that
> allows it to be used for all of the output formats (text, XML, and
> JSON).  I think it we should be looking for a way to allow modules to
> publish abstract objects like property-value mappings, or lists of
> strings, rather than thinking strictly in terms of XML.  If we have a
> module called foo that emits property bar with value baz and property
> bletch with value quux, then ...

This seems to be missing the point I was trying to make, which is that
a design like that actually offers no leverage at all: if you don't know
all about foo to start with, you have no idea what to do with either bar
or bletch.  You can *parse* the data, since it's in XML or JSON or
whatever, but you don't know what it is.

The EXPLAIN problem is a fairly constrained universe: there is going to
be a tree of plan nodes, there are going to be some static properties of
each plan node, and there may or may not be various sorts of estimates
and/or measurements attached to each one.  What I'm after is that code
examining the output can know "oh, this is a measurement" even if it
hasn't heard of the particular kind of measurement.

As a concrete example of what I'm thinking about, I'd hope that PgAdmin
would be able to display a graphical summary of a plan tree, and then
pop up measurements associated with one of the nodes when you
right-click on that node.  To do this, it doesn't necessarily have to
know all about each specific measurement that a particular backend
version might emit; but it needs to be able to tell which things are
measurements.
        regards, tom lane