Comparison with Databases - Hadoop

Having seen Pig in action, it might seem that Pig Latin is similar to SQL. The presence of such operators as GROUP BY and DESCRIBE reinforces this impression. However, there are several differences between the two languages, and between Pig and RDBMSs in general

The most significant difference is that Pig Latin is a data flow programming language, whereas SQL is a declarative programming language. In other words, a Pig Latin program is a step-by-step set of operations on an input relation, in which each step is a single transformation. By contrast, SQL statements are a set of constraints that, taken together, define the output. In many ways, programming in Pig Latin is like working at the level of an RDBMS query planner, which figures out how to turn a declarative statement into a system of steps.

RDBMSs store data in tables, with tightly predefined schemas. Pig is more relaxed about the data that it processes: you can define a schema at runtime, but it’s optional. Essentially, it will operate on any source of tuples (although the source should support being read in parallel, by being in multiple files, for example), where a UDF is used to read the tuples from their raw representation. The most common representation is atext file with tab-separated fields, and Pig provides a built-in load function for this format.

Unlike with a traditional database, there is no data import process to load the data into the RDBMS. The data is loaded from the filesystem (usually HDFS) as the first step in the processing.

Pig’s support for complex, nested data structures differentiates it from SQL, which operates on flatter data structures. Also, Pig’s ability to use UDFs and streaming operators that are tightly integrated with the language and Pig’s nested data structures makes Pig Latin more customizable than most SQL dialects.

There are several features to support online, low-latency queries that RDBMSs have that are absent in Pig, such as transactions and indexes. As mentioned earlier, Pig does not support random reads or queries in the order of tens of milliseconds. Nor does it support random writes to update small portions of data; all writes are bulk, streaming writes, just like MapReduce.

Hive (covered in Hive Chapter ) sits between Pig and conventional RDBMSs. Like Pig, Hive is designed to use HDFS for storage, but otherwise there are some significant differences. Its query language, HiveQL, is based on SQL, and anyone who is familiar with SQL would have little trouble writing queries in HiveQL. Like RDBMSs, Hive mandates that all data be stored in tables, with a schema under its management; however,it can associate a schema with preexisting data in HDFS, so the load step is optional. Hive does not support low-latency queries, a characteristic it shares with Pig.

Or as the Pig Philosophy has it, “Pigs eat anything.”

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