Hi Ted,
Thanks much for the feedback! Good test cases indeed. The good news is that
we're close to finishing a "V2" JSON reader that smooths over a few more JSON
quirks like the "first column null" issue that can cause problems:
{n: 1, v: null}{n: 2, v: "Gotcha! Wasn't Int, is actually VARCHAR!"}
Glad your queries work. You gave an example that had fooled me multiple times:
select nest.u from dfs.root.`/Users/tdunning/x.json`;
The trick here is that Drill has no schema. All the parser can tell is, "hey,
I've got a two-part name, `nest.u`. For me a two part name means schema.table
or table.column, so, since `nest` isn't a schema, it must be a table.Oh, look,
no such table exists. FAIL!" Using a three-part name works (IIRC):
select t.nest.u from dfs.root.`/Users/tdunning/x.json` t;
Now Drill sees that `t` is a table name, and works its way down from there.
If Drill had a schema, then the planner could first check if `nest` is a
schema, then if it is a table, then if it is a structured field in the query.
Impala can do this because it has a schema; Drill can't. We can hope that, with
the new schema work being added to Drill, that your query will "do the right
thing" in the future.
Adding `columns` to your query won't help: the `columns` name is valid in only
one place: when working with CSV (or, more generally, delimited) data with no
headers.
This gets back to Jaing's point: we could really use better/more documentation.
We're good at the bare basics, "such-and-so syntax exists", but we're not as
good at explaining how to solve problems using Drill features. The Learning
Apache Drill book tries to address some holes. Clearly, if you have a hard time
with this, being part of the team that created Drill, we've got a bit of work
to do! (To be honest, neither Impala nor Presto are much better in the "how to"
department.)
Additional use cases/frustrations are very welcome as you find them.
Thanks,
- Paul
On Wednesday, January 15, 2020, 3:44:09 PM PST, Ted Dunning
<[email protected]> wrote:
On Wed, Jan 15, 2020 at 2:58 PM Paul Rogers <[email protected]>
wrote:
> ...
>
> For example, Ted, you mention lack of nullability on structure members.
> But, Drill represents structures as MAPs, and MAPs can have nullable
> members. So, there is likely more to your request than the short summary
> suggests. Perhaps you can help us understand this a bit more.
>
This was quite a while ago.
I was reading JSON data with substructures of variable form.
I think, however, that this impression is old news. I just tried it and it
works the way I wanted.
Here is my data:
{"top":"a","nest":{"u":1, "v":"other"}}
{"top":"b","nest":{"v":"this", "w":"that"}}
And here are some queries that behave just the way that I wanted:
apache drill> *select* * *from* dfs.root.`/Users/tdunning/x.json`;
+-----+-------------------------+
| *top* | * nest * |
+-----+-------------------------+
| a | {"u":1,"v":"other"} |
| b | {"v":"this","w":"that"} |
+-----+-------------------------+
2 rows selected (0.079 seconds)
apache drill> *select* nest *from* dfs.root.`/Users/tdunning/x.json`;
+-------------------------+
| * nest * |
+-------------------------+
| {"u":1,"v":"other"} |
| {"v":"this","w":"that"} |
+-------------------------+
2 rows selected (0.114 seconds)
apache drill> *select* nest.u *from* dfs.root.`/Users/tdunning/x.json`;
Error: VALIDATION ERROR: From line 1, column 8 to line 1, column 11: Table
'nest' not found
[Error Id: b2100faf-adf7-453e-957f-56726b96e06f ] (state=,code=0)
apache drill> *select* columns.nest.u *from* dfs.root.
`/Users/tdunning/x.json`;
Error: VALIDATION ERROR: From line 1, column 8 to line 1, column 14: Table
'columns' not found
[Error Id: a793e6bd-c2ed-477a-9f23-70d67b2b85df ] (state=,code=0)
apache drill> *select* x.nest.u *from* dfs.root.`/Users/tdunning/x.json` x;
+--------+
| *EXPR$0* |
+--------+
| 1 |
| null |
+--------+
2 rows selected (0.126 seconds)
apache drill>
