Some Basic Hive Commands for Sanity's Sake
[hive python aws wwe con = connect_to_hive()
ex = con.execute
from pandas import read_sql_query as qry
These commands work with either a Hive connection. Some of them work with a Presto connection, but not all. Presto has different syntax, which I’ll save for another day.
# List available hive databases
qry('SHOW DATABASES', con)
# List tables
qry('SHOW TABLES', con)
# Show Table Info (e.g., column data types)
qry('DESCRIBE table_name', con)
Create an External Table from CSV Files in S3
If you collect a lot of data in S3, keep it there! For a long time, my team was using S3 as a holding site, ultimately waiting to be loaded into Redshift. But time passed and we grew smarter. Not saying Redshift doesn’t have its use cases, but then again, you can just toss CSV files into an S3 directory and treat it as a table in Hive. Better, once the Hive table is created, you can hit it with Presto, which is faster (maybe you can even create the Hive table while in Presto…not sure).
CREATE EXTERNAL TABLE table_name (col1 dataType1, …, colN dataTypeN)
ROW FORMAT DELIMITED FIELDS TERMINATED BY ','
LOCATION 's3://bucketname/keypath/'
TBLPROPERTIES ('skip.header.line.count'='1')
There are some important hiccups I ran into.
- there is no datetime data type in Hive
- I’m actually not mapping the CSV file to a Hive table successfully (some columns are skipped…?)
Features I need to figure out
Obviously, the table issue above for starters.
Also, our file naming scheme has the as_on_date in the filename, and I’d love to get this into a column in the Hive table. Related to this, I found that every Hive table has a “virutal column” that stores what file name/location a row comes from:
SELECT INPUT__FILE__NAME FROM tbl_name
If you know the filename, you can pluck out the date, like so:
qry("select substr(INPUT__FILE__NAME,-19,8) from tbl_name limit 5",con)
But, of course, all this I’m doing after the table was already created… Not the biggest deal, but still kind of annoying.