Useful queries for the Hive metastore

The Hive metastore stores metadata about objects within Hive.  Usually this metastore sits within a relational database such as MySQL. Sometimes it’s useful to query the Hive metastore directly to find out what databases, tables and views exist in Hive and how they’re defined. For example, say we want to expose a report to users…

Python + JDBC = Dynamic Hive scripting

Working with Hive can be challenging without the benefit of a procedural language (such as T-SQL or PL/SQL) in order to do things with data in between Hive statements or run dynamic hive statements in bulk.  For example – we may want to do a rowcount of all tables in one of our Hive databases,…

Useful date formulas for Hive

Hive comes with some handy functions for transforming dates.  These can be helpful when working with date dimension tables and performing time-based comparisons and aggregations. e.g. Convert a native Hive date formatted date string: date_format(myDate,’dd-MM-yyyy’) Return the week number (within the year) of a particular date – i.e. first week of the year is 1,…

Finding a sequence of events in Hive using analytic functions

Hadoop Hive features several useful functions for efficiently performing analytics over ordered sets of rows — these are known as the windowing and analytics functions.  For example, lead and lag functions can be used to produce rolling averages or cumulative sums over a window of time (e.g. hourly averages for some metric over the preceeding…

Using HDFS path and filename as columns in a Hive table

A handy feature of Hadoop Hive is the ability to use the filename and path of underlying files as columns in a view or table using the virtual Hive column INPUT__FILE__NAME.  This is particularly handy in the case of external tables where some metadata about files is embedded in the location on HDFS or the…

Creating a custom Hive input format and record reader to read fixed format flat files

Apache Hive is great for enabling SQL-like queryability over flat files.  This is trivial in the case of tabular formatted files such as CSV files where we can set custom row and field delimiters out-of-the-box (e.g. Hive’s inbuilt CSV serde).  Even more complex files can be read and converted to a desired row and column…

Selecting maximum of two columns – timestamp and sequence in Hive

Sometimes it is useful to sort data by two columns and get the maximum of both columns for each record (column A then column B). An example is a table with a logical primary key and an additional timestamp field and a sequence number field.  This could be the case, for example if data is…