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03 hive query language (hql)

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03 hive query language (hql)

  1. 1. Hive Query Language (HQL)
  2. 2. Example • Here’s one way of populating a single row table: % echo 'X' > /tmp/dummy.txt % hive -e "CREATE TABLE dummy (value STRING); LOAD DATA LOCAL INPATH '/tmp/dummy.txt' OVERWRITE INTO TABLE dummy" • Use ‘hive’ to start hive • SHOW TABLES; • SELECT * FROM dummy; • DROP TABLE dummy; • SHOW TABLES;
  3. 3. Example • Let’s see how to use Hive to run a query on the weather dataset • The first step is to load the data into Hive’s managed storage. • Here we’ll have Hive use the local file-system for storage; later we’ll see how to store tables in HDFS. • Just like an RDBMS, Hive organizes its data into tables. • We create a table to hold the weather data using the CREATE TABLE statement: CREATE TABLE records (year STRING, temperature INT, quality INT) ROW FORMAT DELIMITED FIELDS TERMINATED BY 't'; • The first line declares a records table with three columns: year, temperature, and quality. • The type of each column must be specified, too: here the year is a string, while the other two columns are integers. • So far, the SQL is familiar. The ROW FORMAT clause, however, is particular to HiveQL. • It is saying that each row in the data file is tab-delimited text.
  4. 4. Example • Next we can populate Hive with the data. This is just a small sample, for exploratory purposes: LOAD DATA LOCAL INPATH ‘input/ncdc/micro-tab/sample.txt' OVERWRITE INTO TABLE records; • Running this command tells Hive to put the specified local file in its warehouse directory. • This is a simple filesystem operation. • There is no attempt, for example, to parse the file and store it in an internal database format, since Hive does not mandate any particular file format. Files are stored verbatim: they are not modified by Hive. • Tables are stored as directories under Hive’s warehouse directory, which is controlled by the hive.metastore.warehouse.dir, and defaults to /user/hive/warehouse • The OVERWRITE keyword in the LOAD DATA statement tells Hive to delete any existing files in the directory for the table. If it is omitted, then the new files are simply added to the table’s directory (unless they have the same names, in which case they replace the old files).
  5. 5. Example • Query: SELECT year, MAX(temperature) FROM records WHERE temperature != 9999 AND (quality = 0 OR quality = 1 OR quality = 4 OR quality = 5 OR quality = 9) GROUP BY year; • Hive is configured using an XML configuration file like Hadoop’s. The file is called hive-site.xml and is located in Hive’s conf directory.
  6. 6. Hive Services • cli – The command line interface to Hive (the shell). This is the default service. • hiveserver – Runs Hive as a server exposing a Thrift service, enabling access from a range of clients written in different languages. Applications using the Thrift, JDBC, and ODBC connectors need to run a Hive server to communicate with Hive. Set the HIVE_PORT environment variable to specify the port the server will listen on (defaults to 10,000). • hwi – The Hive Web Interface. • jar – The Hive equivalent to hadoop jar, a convenient way to run Java applications that includes both Hadoop and Hive classes on the classpath. • metastore – By default, the metastore is run in the same process as the Hive service. Using this service, it is possible to run the metastore as a standalone (remote) process. Set the METASTORE_PORT environment variable to specify the port the server will listen on.
  7. 7. Hive architecture
  8. 8. Clients • Thrift Client – The Hive Thrift Client makes it easy to run Hive commands from a wide range of programming languages. Thrift bindings for Hive are available for C++, Java, PHP, Python, and Ruby. They can be found in the src/service/src subdirectory in the Hive distribution. • JDBC Driver – Hive provides a Type 4 (pure Java) JDBC driver, defined in the class org.apache.hadoop.hive.jdbc.HiveDriver. When configured with a JDBC URI of the form jdbc:hive://host:port/dbname, a Java application will connect to a Hive server running in a separate process at the given host and port. (The driver makes calls to an interface implemented by the Hive Thrift Client using the Java Thrift bindings.) You may alternatively choose to connect to Hive via JDBC in embedded mode using the URI jdbc:hive://. In this mode, Hive runs in the same JVM as the application invoking it, so there is no need to launch it as a standalone server since it does not use the Thrift service or the Hive Thrift Client. • ODBC Driver – The Hive ODBC Driver allows applications that support the ODBC protocol to connect to Hive. (Like the JDBC driver, the ODBC driver uses Thrift to communicate with the Hive server.) The ODBC driver is still in development, so you should refer to the latest instructions on the Hive wiki for how to build and run it.
  9. 9. The Metastore (Embedded) • The metastore is the central repository of Hive metadata. The metastore is divided into two pieces: – a service and – the backing store for the data • By default, the metastore service runs in the same JVM as the Hive service and contains an embedded Derby database instance backed by the local disk. • This is called the embedded metastore configuration. • Using an embedded metastore is a simple way to get started with Hive; however, only one embedded Derby database can access the database files on disk at any one time, which means you can only have one Hive session open at a time that shares the same metastore.
  10. 10. Local Metastore • The solution to supporting multiple sessions (and therefore multiple users) is to use a standalone database. • This configuration is referred to as a local metastore, since the metastore service still runs in the same process as the Hive service, but connects to a database running in a separate process, either on the same machine or on a remote machine. • Any JDBC-compliant database may be used. • MySQL is a popular choice for the standalone metastore. • In this case, javax.jdo.option.ConnectionURL is set to jdbc:mysql://host/dbname?createDatabaseIfNotExist=true, and javax.jdo.option.ConnectionDriverName is set to com.mysql.jdbc.Driver. (The user name and password should be set, too, of course.)
  11. 11. Remote Metastore • Going a step further, there’s another metastore configuration called a remote metastore, where one or more metastore servers run in separate processes to the Hive service. • This brings better manageability and security, since the database tier can be completely firewalled off, and the clients no longer need the database credentials.
  12. 12. Updates, transactions, and indexes • Updates, transactions, and indexes are mainstays of traditional databases. • Yet, until recently, these features have not been considered a part of Hive’s feature set. • This is because Hive was built to operate over HDFS data using MapReduce, where full-table scans are the norm and a table update is achieved by transforming the data into a new table. • For a data warehousing application that runs over large portions of the dataset, this works well.
  13. 13. HiveQL or HQL • Hive’s SQL dialect, called HiveQL, does not support the full SQL-92 specification.
  14. 14. Tables • A Hive table is logically made up of the data being stored and the associated metadata describing the layout of the data in the table. • The data typically resides in HDFS, although it may reside in any Hadoop filesystem, including the local filesystem or S3. • Hive stores the metadata in a relational database—and not in HDFS, say Metastore. • Many relational databases have a facility for multiple namespaces, which allow users and applications to be segregated into different databases or schemas. • Hive supports the same facility, and provides commands such as CREATE DATABASE dbname, USE dbname, and DROP DATABASE dbname. • You can fully qualify a table by writing dbname.tablename. If no database is specified, tables belong to the default database.
  15. 15. Managed Tables and External Tables • When you create a table in Hive, by default Hive will manage the data, which means that Hive moves the data into its warehouse directory. Alternatively, you may create an external table, which tells Hive to refer to the data that is at an existing location outside the warehouse directory. • The difference between the two types of table is seen in the LOAD and DROP semantics. • When you load data into a managed table, it is moved into Hive’s warehouse directory. • CREATE EXTERNAL TABLE • With the EXTERNAL keyword, Hive knows that it is not managing the data, so it doesn’t move it to its warehouse directory. • Hive organizes tables into partitions, a way of dividing a table into coarse-grained parts based on the value of a partition column, such as date. Using partitions can make it faster to do queries on slices of the data. • CREATE TABLE logs (ts BIGINT, line STRING) PARTITIONED BY (dt STRING, country STRING); • LOAD DATA LOCAL INPATH 'input/hive/partitions/file1‘ INTO TABLE logs PARTITION (dt='2001-01-01', country='GB'); • SHOW PARTITIONS logs;
  16. 16. Buckets • There are two reasons why you might want to organize your tables (or partitions) into buckets. • The first is to enable more efficient queries. • Bucketing imposes extra structure on the table, which Hive can take advantage of when performing certain queries. • In particular, a join of two tables that are bucketed on the same columns—which include the join columns—can be efficiently implemented as a map-side join.
  17. 17. Buckets • The second reason to bucket a table is to make sampling more efficient. • When working with large datasets, it is very convenient to try out queries on a fraction of your dataset while you are in the process of developing or refining them. • CREATE TABLE bucketed_users (id INT, name STRING) CLUSTERED BY (id) INTO 4 BUCKETS; • Any particular bucket will effectively have a random set of users in it. • The data within a bucket may additionally be sorted by one or more columns. This allows even more efficient map-side joins, since the join of each bucket becomes an efficient merge-sort. • CREATE TABLE bucketed_users (id INT, name STRING) CLUSTERED BY (id) SORTED BY (id ASC) INTO 4 BUCKETS;
  18. 18. Buckets • Physically, each bucket is just a file in the table (or partition) directory. • In fact, buckets correspond to MapReduce output file partitions: a job will produce as many buckets (output files) as reduce tasks. • We can do a sampling of the table using the TABLESAMPLE clause, which restricts the query to a fraction of the buckets in the table rather than the whole table: • SELECT * FROM bucketed_users TABLESAMPLE(BUCKET 1 OUT OF 4 ON id); • Bucket numbering is 1-based, so this query retrieves all the users from the first of four buckets. • For a large, evenly distributed dataset, approximately one quarter of the table’s rows would be returned.
  19. 19. Storage Formats • There are two dimensions that govern table storage in Hive: the row format and the file format. • The row format dictates how rows, and the fields in a particular row, are stored. • In Hive parlance, the row format is defined by a SerDe. • When you create a table with no ROW FORMAT or STORED AS clauses, the default format is delimited text, with a row per line. • You can use sequence files in Hive by using the declaration STORED AS SEQUENCEFILE in the CREATE TABLE statement. • Hive provides another binary storage format called RCFile, short for Record Columnar File. RCFiles are similar to sequence files, except that they store data in a columnoriented fashion.
  20. 20. Lets do some Hands on • Look at the data in /input/tsv/, and choose any one of the TSV file • Understand the col types and names • drop table X; drop table bucketed_X; • CREATE TABLE X(id STRING, name STRING) • ROW FORMAT DELIMITED • FIELDS TERMINATED BY 't'; • LOAD DATA LOCAL INPATH '/input/tsv/X.tsv‘ OVERWRITE INTO TABLE X; • SELECT * FROM X; • Write some query : SELECT x • FROM X • WHERE y == ABC • GROUP BY z; • dfs -cat /user/hive/warehouse/X/X.tsv; • CREATE TABLE bucketed_X (id STRING, name STRING) CLUSTERED BY (id) INTO 4 BUCKETS; • CREATE TABLE bucketed_X (id STRING, name STRING) CLUSTERED BY (id) SORTED BY (id) INTO 4 BUCKETS; • SELECT * FROM X;
  21. 21. Lets do some Hands on • SET hive.enforce.bucketing=true; • INSERT OVERWRITE TABLE bucketed_X SELECT * FROM X; • dfs -ls /user/hive/warehouse/bucketed_X; • SELECT * FROM bucketed_X TABLESAMPLE(BUCKET 1 OUT OF 4 ON id); • SELECT * FROM bucketed_X TABLESAMPLE(BUCKET 1 OUT OF 2 ON id); • SELECT * FROM X TABLESAMPLE(BUCKET 1 OUT OF 4 ON rand());
  22. 22. Querying Data • Sorting data in Hive can be achieved by use of a standard ORDER BY clause, but there is a catch. • ORDER BY produces a result that is totally sorted, as expected, but to do so it sets the number of reducers to one, making it very inefficient for large datasets. • When a globally sorted result is not required—and in many cases it isn’t— then you can use Hive’s nonstandard extension, SORT BY instead. • SORT BY produces a sorted file per reducer. • In some cases, you want to control which reducer a particular row goes to, typically so you can perform some subsequent aggregation. This is what Hive’s DISTRIBUTE BY clause does. • FROM records SELECT year, temperature DISTRIBUTE BY year SORT BY year ASC, temperature DESC;
  23. 23. MapReduce Scripts • Using an approach like Hadoop Streaming, the TRANSFORM, MAP, and REDUCE clauses make it possible to invoke an external script or program from Hive. #!/usr/bin/env python import re import sys for line in sys.stdin: (year, temp, q) = line.strip().split() if (temp != "9999" and re.match("[01459]", q)): print "%st%s" % (year, temp) is_good_quality.py #!/usr/bin/env python import sys (last_key, max_val) = (None, 0) for line in sys.stdin: (key, val) = line.strip().split("t") if last_key and last_key != key: print "%st%s" % (last_key, max_val) (last_key, max_val) = (key, int(val)) else: (last_key, max_val) = (key, max(max_val, int(val))) if last_key: print "%st%s" % (last_key, max_val) /code/hive/python/max_temperature_reduce.py; • ADD FILE Hadoop_training/code/hive/python/is_good_quality.py; ADD FILE Hadoop_training/code/hive/python/max_temperature_reduce.py;
  24. 24. Example CREATE TABLE records (year STRING, temperature INT, quality INT) ROW FORMAT DELIMITED FIELDS TERMINATED BY 't'; LOAD DATA LOCAL INPATH 'input/ncdc/micro-tab/sample.txt' OVERWRITE INTO TABLE records; FROM records SELECT TRANSFORM(year, temperature, quality) USING 'is_good_quality.py' AS year, temperature; FROM ( FROM records MAP year, temperature, quality USING 'is_good_quality.py' AS year, temperature) map_output REDUCE year, temperature USING 'max_temperature_reduce.py' AS year, temperature;
  25. 25. Joins • The simplest kind of join is the inner join, where each match in the input tables results in a row in the output. • SELECT sales.*, things.* FROM sales JOIN things ON (sales.id = things.id); • Outer Join: • SELECT sales.*, things.* FROM sales LEFT OUTER JOIN things ON (sales.id = things.id); • SELECT sales.*, things.* FROM sales RIGHT OUTER JOIN things ON (sales.id = things.id); • SELECT sales.*, things.* FROM sales FULL OUTER JOIN things ON (sales.id = things.id);
  26. 26. Joins • SELECT * FROM things WHERE things.id IN (SELECT id from sales); • Is not possible in Hive – but can be written as: • SELECT * FROM things LEFT SEMI JOIN sales ON (sales.id = things.id);
  27. 27. End of session Day – 4: Hive Query Language (HQL)

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