How to Hive ----------- DISCLAIMER: This is a prototype version of Hive and is NOT production quality. This is provided mainly as a way of illustrating the capabilities of Hive and is provided as-is. However - we are working hard to make Hive a production quality system. Hive has only been tested on unix(linux) and mac systems using Java 1.6 for now - although it may very well work on other similar platforms. It does not work on Cygwin right now. Most of our testing has been on Hadoop 0.17 - so we would advise running it against this version of hadoop - even though it may compile/work against other versions Downloading and building ------------------------ You can either build hive jar files for your environment: - apply patch - cd hadoop/src/contrib/hive - src/contrib/hive> ant -Dtarget.dir= package Or you can use the pre-packaged jars that is available at the following location and untar it: http://mirror.facebook.com/facebook/hive/hadoop-0.17/dist.tar.gz $ wget http://mirror.facebook.com/facebook/hive/hadoop-0.17/dist.tar.gz $ tar zxvf dist.tar.gz $ cd dist $ ls README bin conf lib bin/ (all the shell scripts) lib/ (required jar files) conf/ (configuration files) In the rest of the README, we use dist and interchangeably. Running Hive ------------ Hive uses hadoop that means: - you must have hadoop in your path OR - export HADOOP=/bin/hadoop To use hive command line interface (cli) from the shell: $ bin/hive Using Hive ---------- Configuration management overview --------------------------------- - hive configuration is stored in /conf/hive-default.xml and log4j in hive-log4j.properties - hive configuration is an overlay on top of hadoop - meaning the hadoop configuration variables are inherited by default. - hive configuration can be manipulated by: o editing hive-default.xml and defining any desired variables (including hadoop variables) in it o from the cli using the set command (see below) o by invoking hive using the syntax: * bin/hive -hiveconf x1=y1 -hiveconf x2=y2 this sets the variables x1 and x2 to y1 and y2 Error Logs ---------- Hive uses log4j for logging. By default logs are not emitted to the console by the cli. They are stored in the file: - /tmp/{user.name}/hive.log If the user wishes - the logs can be emitted to the console by adding the arguments shown below: - bin/hive -hiveconf hive.root.logger=INFO,console Note that setting hive.root.logger via the 'set' command does not change logging properties since they are determined at initialization time. Error logs are very useful to debug problems. Please send them with any bugs (of which there are many!) to athusoo@facebok.com. DDL Operations -------------- Creating Hive tables and browsing through them hive> CREATE TABLE pokes (foo INT, bar STRING); Creates a table called pokes with two columns, first being an integer and other a string columns hive> CREATE TABLE invites (foo INT, bar STRING) PARTITIONED BY (ds STRING); Creates a table called invites with two columns and a partition column called ds. The partition column is a virtual column it is not part of the data itself but is derived from the partition that a particular dataset is loaded into. By default tables are assumed to be of text input format and the delimiters are assumed to be ^A(ctrl-a). We will be soon publish additional commands/recipes to add binary (sequencefiles) data and configurable delimiters etc. hive> SHOW TABLES; lists all the tables hive> SHOW TABLES '.*s'; lists all the table that end with 's'. The pattern matching follows Java regular expressions. Check out this link for documentation http://java.sun.com/javase/6/docs/api/java/util/regex/Pattern.html hive> DESCRIBE invites; shows the list of columns Altering tables. Table name can be changed and additional columns can be dropped hive> ALTER TABLE pokes ADD COLUMNS (new_col INT); hive> ALTER TABLE invites ADD COLUMNS (new_col2 INT COMMENT 'a comment'); hive> ALTER TABLE events RENAME TO 3koobecaf; Dropping tables hive> DROP TABLE pokes; Metadata Store -------------- Metadata is in an embedded Derby database whose location is determined by the hive configuration variable named javax.jdo.option.ConnectionURL. By default (see conf/hive-default.xml) - this location is ./metastore_db Right now - in the default configuration, this metadata can only be seen by one user at a time. Metastore can be stored in any database that is supported by JPOX. The location and the type of the RDBMS can be controlled by the two variables 'javax.jdo.option.ConnectionURL' and 'javax.jdo.option.ConnectionDriverName'. Refer to JDO (or JPOX) documentation for more details on supported databases. The database schema is defined in JDO metadata annotations file package.jdo at src/contrib/hive/metastore/src/model. In the future - the metastore itself can be a standalone server. DML Operations -------------- Loading data from flat files into Hive hive> LOAD DATA LOCAL INPATH './examples/files/kv1.txt' OVERWRITE INTO TABLE pokes; Loads a file that contains two columns separated by ctrl-a into pokes table. 'local' signifies that the input file is on the local system. If 'local' is omitted then it looks for the file in HDFS. The keyword 'overwrite' signifies that existing data in the table is deleted. If the 'overwrite' keyword is omitted - then data files are appended to existing data sets. NOTES: - NO verification of data against the schema - if the file is in hdfs it is moved into hive controlled file system namespace. The root of the hive directory is specified by the option hive.metastore.warehouse.dir in hive-default.xml. We would advise that this directory be pre-existing before trying to create tables via Hive. hive> LOAD DATA LOCAL INPATH './examples/files/kv2.txt' OVERWRITE INTO TABLE invites PARTITION (ds='2008-08-15'); hive> LOAD DATA LOCAL INPATH './examples/files/kv3.txt' OVERWRITE INTO TABLE invites PARTITION (ds='2008-08-08'); The two LOAD statements above load data into two different partitions of the table invites. Table invites must be created as partitioned by the key ds for this to succeed. Loading/Extracting data using Queries ------------------------------------- Runtime configuration --------------------- - Hives queries are executed using map-reduce queries and as such the behavior of such queries can be controlled by the hadoop configuration variables - The cli can be used to set any hadoop (or hive) configuration variable. For example: o hive> SET mapred.job.tracker=myhost.mycompany.com:50030 o hive> SET - v The latter shows all the current settings. Without the v option only the variables that differ from the base hadoop configuration are displayed - In particular the number of reducers should be set to a reasonable number to get good performance (the default is 1!) EXAMPLE QUERIES --------------- Some example queries are shown below. They are available in examples/queries. More are available in the hive contrib sources src/test/queries/positive SELECTS and FILTERS ------------------- hive> SELECT a.foo FROM invites a; select column 'foo' from all rows of invites table. The results are not stored anywhere, but are displayed on the console. Note that in all the examples that follow, INSERT (into a hive table, local directory or HDFS directory) is optional. hive> INSERT OVERWRITE DIRECTORY '/tmp/hdfs_out' SELECT a.* FROM invites a; select all rows from invites table into an HDFS directory. The result data is in files (depending on the number of mappers) in that directory. NOTE: partition columns if any are selected by the use of *. They can also be specified in the projection clauses. hive> INSERT OVERWRITE LOCAL DIRECTORY '/tmp/local_out' SELECT a.* FROM pokes a; Select all rows from pokes table into a local directory hive> INSERT OVERWRITE TABLE events SELECT a.* FROM profiles a; hive> INSERT OVERWRITE TABLE events SELECT a.* FROM profiles a WHERE a.key < 100; hive> INSERT OVERWRITE LOCAL DIRECTORY '/tmp/reg_3' SELECT a.* FROM events a; hive> INSERT OVERWRITE DIRECTORY '/tmp/reg_4' select a.invites, a.pokes FROM profiles a; hive> INSERT OVERWRITE DIRECTORY '/tmp/reg_5' SELECT COUNT(1) FROM invites a; hive> INSERT OVERWRITE DIRECTORY '/tmp/reg_5' SELECT a.foo, a.bar FROM invites a; hive> INSERT OVERWRITE LOCAL DIRECTORY '/tmp/sum' SELECT SUM(a.pc) FROM pc1 a; Sum of a column. avg, min, max can also be used NOTE: there are some flaws with the type system that cause doubles to be returned with integer types would be expected. We expect to fix these in the coming week. GROUP BY -------- hive> FROM invites a INSERT OVERWRITE TABLE events SELECT a.bar, count(1) WHERE a.foo > 0 GROUP BY a.bar; hive> INSERT OVERWRITE TABLE events SELECT a.bar, count(1) FROM invites a WHERE a.foo > 0 GROUP BY a.bar; NOTE: Currently Hive always uses two stage map-reduce for groupby operation. This is to handle skews in input data. We will be optimizing this in the coming weeks. JOIN ---- hive> FROM pokes t1 JOIN invites t2 ON (t1.bar = t2.bar) INSERT OVERWRITE TABLE events SELECT t1.bar, t1.foo, t2.foo MULTITABLE INSERT ----------------- FROM src INSERT OVERWRITE TABLE dest1 SELECT src.* WHERE src.key < 100 INSERT OVERWRITE TABLE dest2 SELECT src.key, src.value WHERE src.key >= 100 and src.key < 200 INSERT OVERWRITE TABLE dest3 PARTITION(ds='2008-04-08', hr='12') SELECT src.key WHERE src.key >= 200 and src.key < 300 INSERT OVERWRITE LOCAL DIRECTORY '/tmp/dest4.out' SELECT src.value WHERE src.key >= 300 STREAMING --------- hive> FROM invites a INSERT OVERWRITE TABLE events SELECT TRANSFORM(a.foo, a.bar) AS (oof, rab) USING '/bin/cat' WHERE a.ds > '2008-08-09'; This streams the data in the map phase through the script /bin/cat (like hadoop streaming). Similarly - streaming can be used on the reduce side (please see the Hive Tutorial or examples) KNOWN BUGS/ISSUES ----------------- * hive cli may hang for a couple of minutes because of a bug in getting metadata from the derby database. let it run and you'll be fine! * hive cli does not support multi-line queries. * hive cli creates derby.log in the directory from which it has been invoked. * DESCRIBE table currently only shows columns in a table. Other metadata like partitions, buckets etc are not shown. * LOAD FILE or INSERT INTO TABLE do not validate schemas of the destination tables. * COUNT(*) does not work for now. Use COUNT(1) instead. * String literals are indicated by single quotes(double quotes are not supported). So 'is a valid string' while "is not a valid string" in the query language. Hive does support escaping quotes and semi-colon similar to MySQL. * Multiple GROUP BYs are not supported in the multi-table table INSERT queries. * ORDER BY not supported. * Only string and thrift types (http://developers.facebook.com/thrift) have been tested. FUTURE FEATURES ---------------