How to query json file located at s3 using presto - json

Question

I have one json file stored in amazon-s3 location, I want to query this json file using presto. how can I achieve this?

Answer 1

Option 1 - Presto on EMR with json_extract built-in function
I am supposing that you have already launched Presto using EMR.
The easiest way to do this would be to use the json_extract function that comes by default with Presto.
So imagine you have a json file on s3 like this:
{"a": "a_value1", "b": { "bb": "bb_value1" }, "c": "c_value1"}
{"a": "a_value2", "b": { "bb": "bb_value2" }, "c": "c_value2"}
{"a": "a_value3", "b": { "bb": "bb_value3" }, "c": "c_value3"}
...
...
Each row represents a json tree object.
So you can simply define in presto a table with one field which is of string type, and then easily query the table with json_extract.
SELECT json_extract(json_field, '$.b.bb') as extract
FROM my_table
The result would be something like:
| extract |
|-----------------|
| bb_value1 |
| bb_value2 |
| bb_value3 |
This can be a fast and easy way to read a json file using presto, but unfortunately this doesn't scale well on big json files.
Some presto docs on json_extract: https://prestodb.github.io/docs/current/functions/json.html#json_extract
Option 2 - Presto on EMR with a specific Serde for json files
You can also customize your presto in bootstrap phase of your emr cluster, by adding custom plugins or SerDe libraries.
So you just have to choose one of the available JSON SerDe libraries (e.g. org.openx.data.jsonserde.JsonSerDe) and follow their guide to define a table that matches the structure of the Json file.
You will be able to access to the fields of the json file in a similar way of the json_extract (using the dotted notation), and it should be faster and scale well on big files. Unfortunately using this method you have 2 main problems:
1) Defining a table for complex files is like being in hell.
2) You may have internal java cast exception, because the data in the json couldn't be easily casted by the SerDe library.
Option 3 - Athena Built-In JSON Serde
https://docs.aws.amazon.com/athena/latest/ug/json.html
It seems that you have some Json SerDe also built-in Athena, I have personally never tried these but they are managed by AWS so should be easier to set up everything.

Answer 2

Rather than installing and running your own Presto service, there are some other options you can try:
Amazon Athena is a fully-managed Presto service. You can use it to query large datastores in Amazon S3, including compressed and partitioned data.
Amazon S3 Select allows you to run a query on a single object stored in Amazon S3. This is possibly simpler for your particular use-case.