Discussion: Support explain(terse) for queries

[kwannoel]

Sometimes we just want to get the structure of the query plan, e.g. just see the plan nodes, to quickly check number of joins, type of joins, type of aggregations.

├─StreamHashAgg
│ └─StreamShare
│   └─StreamExchange
│     └─StreamTableScan

We can support explain(terse) for this purpose.

Not sure if it will cause a lot of burden for our optimizer. If it does, it's not worth it.

To support this is straightforward:

1. We should reject verbose and terse being set together.
2. For terse, inside explain, rewrite the XMLNode, we can just keep the Record name. Then for the fields, only keep the following:

Join -> `type`, `predicate`
TableScan -> `table`
Agg -> `aggs`, `group_key`

[kwannoel]

I think still useful, although it can already be supported via jq + explain (json).