直接使用来自 Databricks 的原始 SQL 查询存储在 Azure Data Lake 中的 avro 数据文件

Question

I'm using Databricks Notebooks to read avro files stored in an Azure Data Lake Gen2. The avro files are created by an Event Hub Capture, and present a specific schema. From these files I have to extract only the Body field, where the data which I'm interested in is actually stored.

I already implented this in Python and it works as expected:

path = 'abfss://file_system@storage_account.dfs.core.windows.net/root/YYYY/MM/DD/HH/mm/file.avro'
df0 = spark.read.format('avro').load(path) # 1
df1 = df0.select(df0.Body.cast('string')) # 2
rdd1 = df1.rdd.map(lambda x: x[0]) # 3
data = spark.read.json(rdd1) # 4

Now I need to translate this to raw SQL in order to filter the data directly in the SQL query. Considering the 4 steps above, steps 1 and 2 with SQL are as follows:

CREATE TEMPORARY VIEW file_avro
USING avro
OPTIONS (path "abfss://file_system@storage_account.dfs.core.windows.net/root/YYYY/MM/DD/HH/mm/file.avro")

WITH body_array AS (SELECT cast(Body AS STRING) FROM file_avro)

SELECT * FROM body_array

With this partial query I get the same as df1 above (step 2 with Python):

Body
[{"id":"a123","group":"0","value":1.0,"timestamp":"2020-01-01T00:00:00.0000000"},
{"id":"a123","group":"0","value":1.5,"timestamp":"2020-01-01T00:01:00.0000000"},
{"id":"a123","group":"0","value":2.3,"timestamp":"2020-01-01T00:02:00.0000000"},
{"id":"a123","group":"0","value":1.8,"timestamp":"2020-01-01T00:03:00.0000000"}]
[{"id":"b123","group":"0","value":2.0,"timestamp":"2020-01-01T00:00:01.0000000"},
{"id":"b123","group":"0","value":1.2,"timestamp":"2020-01-01T00:01:01.0000000"},
{"id":"b123","group":"0","value":2.1,"timestamp":"2020-01-01T00:02:01.0000000"},
{"id":"b123","group":"0","value":1.7,"timestamp":"2020-01-01T00:03:01.0000000"}]
...

I need to know how to introduce the steps 3 and 4 into the SQL query, to parse the strings into json objects and finally get the desired dataframe with columns id, group, value and timestamp. Thanks.

Answer 1

One way I found to do this with raw SQL is as follows, using from_json Spark SQL built-in function and the scheme of the Body field:

CREATE TEMPORARY VIEW file_avro
USING avro
OPTIONS (path "abfss://file_system@storage_account.dfs.core.windows.net/root/YYYY/MM/DD/HH/mm/file.avro")

WITH body_array AS (SELECT cast(Body AS STRING) FROM file_avro),
data1 AS (SELECT from_json(Body, 'array<struct<id:string,group:string,value:double,timestamp:timestamp>>') FROM body_array),
data2 AS (SELECT explode(*) FROM data1),
data3 AS (SELECT col.* FROM data2)
SELECT * FROM data3 WHERE id = "a123"     --FILTERING BY CHANNEL ID

It performs faster than the Python code I posted in the question, surely because of the use of from_json and the scheme of Body to extract data inside it. My version of this approach in PySpark looks as follows:

path = 'abfss://file_system@storage_account.dfs.core.windows.net/root/YYYY/MM/DD/HH/mm/file.avro'
df0 = spark.read.format('avro').load(path)
df1 = df0.selectExpr("cast(Body as string) as json_data")
df2 = df1.selectExpr("from_json(json_data, 'array<struct<id:string,group:string,value:double,timestamp:timestamp>>') as parsed_json")
data = df2.selectExpr("explode(parsed_json) as json").select("json.*")