Re: Final Patch for GROUPING SETS

>>>>> "Marti" == Marti Raudsepp <marti(at)juffo(dot)org> writes:

Marti> Since you were asking for feedback on the EXPLAIN output on
Marti> IRC, I'd weigh in and say that having the groups on separate
Marti> lines would be significantly more readable.

I revisited the explain output a bit and have come up with these
(surrounding material trimmed for clarity):

(text format)

GroupAggregate (cost=1122.39..1197.48 rows=9 width=8)
Group Key: two, four
Group Key: two
Group Key: ()
-> ...

(xml format)

<Plan>
<Node-Type>Aggregate</Node-Type>
<Strategy>Sorted</Strategy>
<Startup-Cost>1122.39</Startup-Cost>
<Total-Cost>1197.48</Total-Cost>
<Plan-Rows>9</Plan-Rows>
<Plan-Width>8</Plan-Width>
<Grouping-Sets>
<Group-Key>
<Item>two</Item>
<Item>four</Item>
</Group-Key>
<Group-Key>
<Item>two</Item>
</Group-Key>
<Group-Key>
</Group-Key>
</Grouping-Sets>
<Plans>...

(json format)

"Plan": {
"Node Type": "Aggregate",
"Strategy": "Sorted",
"Startup Cost": 1122.39,
"Total Cost": 1197.48,
"Plan Rows": 9,
"Plan Width": 8,
"Grouping Sets": [
["two", "four"],
["two"],
[]
],
"Plans": [...]

(yaml format)

- Plan:
Node Type: "Aggregate"
Strategy: "Sorted"
Startup Cost: 1122.39
Total Cost: 1197.48
Plan Rows: 9
Plan Width: 8
Grouping Sets:
- - "two"
- "four"
- - "two"
-
Plans: ...

Opinions? Any improvements?

I'm not entirely happy with what I had to do with the json and
(especially) the YAML output code in order to make this work. There
seemed no obvious way to generate nested unlabelled structures in
either using the existing Explain* functions, and for the YAML case
the best output structure to produce was entirely non-obvious (and
trying to read the YAML spec made my head explode).

Marti> Do you think it would be reasonable to normalize single-set
Marti> grouping sets into a normal GROUP BY?

It's certainly possible, though it would seem somewhat odd to write
queries that way. Either the parser or the planner could do that;
would you want the original syntax preserved in views, or wouldn't
that matter?

Marti> I'd expect GROUP BY () to be fully equivalent to having no
Marti> GROUP BY clause, but there's a difference in explain
Marti> output. The former displays "Grouping Sets: ()" which is odd,
Marti> since none of the grouping set keywords were used.

That's an implementation artifact, in the sense that we preserve the
fact that GROUP BY () was used by using an empty grouping set. Is it
a problem, really, that it shows up that way in explain?

--
Andrew (irc:RhodiumToad)