Difference between Hive internal tables and external tables?

Question

Can anyone tell me the difference between Hive's external table and internal tables. I know the difference comes when dropping the table. I don't understand what you mean by the data and metadata is deleted in internal and only metadata is deleted in external tables. Can anyone explain me in terms of nodes please.

Comorin

Answer 1

Hive has a relational database on the master node it uses to keep track of state. For instance, when you CREATE TABLE FOO(foo string) LOCATION 'hdfs://tmp/';, this table schema is stored in the database.

If you have a partitioned table, the partitions are stored in the database(this allows hive to use lists of partitions without going to the file-system and finding them, etc). These sorts of things are the 'metadata'.

When you drop an internal table, it drops the data, and it also drops the metadata.

When you drop an external table, it only drops the meta data. That means hive is ignorant of that data now. It does not touch the data itself.

Eugenol

ok.. for example i have created an external table.. and i drop it. what happens? what do you mean by the data is not touched? if i give a select * of that table will it display? i am not able to picture the difference. – Comorin

If you drop a table, Hive returns the state it was in before you dropped the table. if you run the query 'select * from foo' after you drop foo, hive will tell you the table does not exist. This is because you have told hive to forget about that table. The data still exists in whatever filesystem it was in before. Think of the metadata as a 'pointer' to where the data is. – Eugenol

So you are telling if i have data in dis location opt/nancy/foo.txt and i load it in the external table and drop it, the metadata is lost but the data in this location opt/nancy/foo.txt remains? – Comorin

OK, now is this location in the HDFS or my local system? If it is in the local system, when i load the data into an internal table and drop the table the file foo.txt will still remain in that location. am i right so far? – Comorin

Hive tables are on a supported filesystem(Hbase, HDFS,S3, etc). I'm assuming you are using "LOAD DATA" to load data from a local file into a hive table? In this case, you are copying the local file into a hive table. When you drop this table, the copy of the data in the internal table will be deleted, but the source file from the "LOAD DATA" command will still be intact. – Eugenol

I have some odd behavior: When I drop an internal table, sometimes the hdfs directory for it remains. But other times it does not. I'm playing with running the hive shell as different users (hdfs and impala), but I haven't found a pattern yet. Is there some kind of refresh daemon that runs periodically? – Spanish

@Eugenol Hi, I came here because I deleted the metastore.db completely but the data remains on hdfs. So when I do show tables nothing is shown. Is there a way to recreate the metadata. – Vocalist

Answer 2

Hive tables can be created as EXTERNAL or INTERNAL. This is a choice that affects how data is loaded, controlled, and managed.

Use EXTERNAL tables when:

1. The data is also used outside of Hive. For example, the data files are read and processed by an existing program that doesn't lock the files.
2. Data needs to remain in the underlying location even after a DROP TABLE. This can apply if you are pointing multiple schemas (tables or views) at a single data set or if you are iterating through various possible schemas.
3. You want to use a custom location such as ASV.
4. Hive should not own data and control settings, dirs, etc., you have another program or process that will do those things.
5. You are not creating table based on existing table (AS SELECT).

Use INTERNAL tables when:

The data is temporary.

You want Hive to completely manage the lifecycle of the table and data.

Fiacre

Courtesy blogs.msdn.microsoft.com/cindygross/2013/02/05/… – Mickens

will creating a INTERNAL table delete the data from HDFS or it makes a copy and uses exclusively for hive leaving the source(HDFS) intact ? – Auricle

@Fiacre Hi, I came here because I deleted the metastore.db completely but the data remains on hdfs. So when I do show tables nothing is shown. Is there a way to recreate the metadata? – Vocalist

Answer 3

To answer you Question :

For External Tables, Hive stores the data in the LOCATION specified during creation of the table(generally not in warehouse directory). If the external table is dropped, then the table metadata is deleted but not the data.

For Internal tables, Hive stores data into its warehouse directory. If the table is dropped then both the table metadata and the data will be deleted.

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For your reference,

Difference between Internal & External tables :

For External Tables -

• External table stores files on the HDFS server but tables are not linked to the source file completely.

• If you delete an external table the file still remains on the HDFS server.

  As an example if you create an external table called “table_test” in HIVE using HIVE-QL and link the table to file “file”, then deleting “table_test” from HIVE will not delete “file” from HDFS.

• External table files are accessible to anyone who has access to HDFS file structure and therefore security needs to be managed at the HDFS file/folder level.

• Meta data is maintained on master node, and deleting an external table from HIVE only deletes the metadata not the data/file.

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For Internal Tables-

• Stored in a directory based on settings in hive.metastore.warehouse.dir, by default internal tables are stored in the following directory “/user/hive/warehouse” you can change it by updating the location in the config file .
• Deleting the table deletes the metadata and data from master-node and HDFS respectively.
• Internal table file security is controlled solely via HIVE. Security needs to be managed within HIVE, probably at the schema level (depends on organization).

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Hive may have internal or external tables, this is a choice that affects how data is loaded, controlled, and managed.

Use EXTERNAL tables when:

• The data is also used outside of Hive. For example, the data files are read and processed by an existing program that doesn’t lock the files.
• Data needs to remain in the underlying location even after a DROP TABLE. This can apply if you are pointing multiple schema (tables or views) at a single data set or if you are iterating through various possible schema.
• Hive should not own data and control settings, directories, etc., you may have another program or process that will do those things.
• You are not creating table based on existing table (AS SELECT).

Use INTERNAL tables when:

• The data is temporary.
• You want Hive to completely manage the life-cycle of the table and data.

Source :

HDInsight: Hive Internal and External Tables Intro

Internal & external tables in Hadoop- HIVE

Acerbity

@CapturedTree But the answer is not correct. "Hive moves data into its warehouse directory. " -this is completely wrong, it does not. Data is being stored in the table location. Does not matter external or managed. – Lea

Answer 4

An internal table data is stored in the warehouse folder, whereas an external table data is stored at the location you mentioned in table creation.

So when you delete an internal table, it deletes the schema as well as the data under the warehouse folder, but for an external table it's only the schema that you will loose.

So when you want an external table back you again after deleting it, can create a table with the same schema again and point it to the original data location. Hope it is clear now.

Oenomel

Answer 5

The only difference in behaviour (not the intended usage) based on my limited research and testing so far (using Hive 1.1.0 -cdh5.12.0) seems to be that when a table is dropped

• the data of the Internal (Managed) tables gets deleted from the HDFS file system
• while the data of the External tables does NOT get deleted from the HDFS file system.

(NOTE: See Section 'Managed and External Tables' in https://cwiki.apache.org/confluence/display/Hive/LanguageManual+DDL which list some other difference which I did not completely understand)

I believe Hive chooses the location where it needs to create the table based on the following precedence from top to bottom

1. Location defined during the Table Creation
2. Location defined in the Database/Schema Creation in which the table is created.
3. Default Hive Warehouse Directory (Property hive.metastore.warehouse.dir in hive.site.xml)

When the "Location" option is not used during the "creation of a hive table", the above precedence rule is used. This is applicable for both Internal and External tables. This means an Internal table does not necessarily have to reside in the Warehouse directory and can reside anywhere else.

Note: I might have missed some scenarios, but based on my limited exploration, the behaviour of both Internal and Extenal table seems to be the same except for the one difference (data deletion) described above. I tried the following scenarios for both Internal and External tables.

1. Creating table with and without Location option
2. Creating table with and without Partition Option
3. Adding new data using the Hive Load and Insert Statements
4. Adding data files to the Table location outside of Hive (using HDFS commands) and refreshing the table using the "MSCK REPAIR TABLE command
5. Dropping the tables

Unquestionable

nice exploration. This is the only answer explained that how hive handles location option for internal and external table. – Comehither

Answer 6

In external tables, if you drop it, it deletes only schema of the table, table data exists in physical location. So to deleted the data use hadoop fs - rmr tablename . Managed table hive will have full control on tables. In external tables users will have control on it.

Effloresce

I'm encountering a situation where the directory is not always deleted after a DROP TABLE on an internal table created via CREATE TABLE foo (id INT). The metadata is apparently ok because SHOW TABLES is consistent - the table does not appear in that list after it's dropped. Confusingly, I've noticed the dir /is/ deleted sometimes, but I can't consistently recreate that. Any ideas? – Spanish

Are the permissions of the tables checked? You may have changed the ownership of the location on HDFS to some different User. – Disinfest

Answer 7

Internal tables are useful if you want Hive to manage the complete lifecycle of your data including the deletion, whereas external tables are useful when the files are being used outside of Hive.

Bircher

Answer 8

External hive table has advantages that it does not remove files when we drop tables,we can set row formats with different settings , like serde....delimited

Dulia

Answer 9

Consider this scenario which best suits for External Table:

A MapReduce (MR) job filters a huge log file to spit out n sub log files (e.g. each sub log file contains a specific message type log) and the output i.e n sub log files are stored in hdfs.

These log files are to be loaded into Hive tables for performing further analytic, in this scenario I would recommend an External Table(s), because the actual log files are generated and owned by an external process i.e. a MR job besides you can avoid an additional step of loading each generated log file into respective Hive table as well.

Stink

Answer 10

Also Keep in mind that Hive is a big data warehouse. When you want to drop a table you dont want to lose Gigabytes or Terabytes of data. Generating, moving and copying data at that scale can be time consuming. When you drop a 'Managed' table hive will also trash its data. When you drop a 'External' table only the schema definition from hive meta-store is removed. The data on the hdfs still remains.

Devlin

Answer 11

The best use case for an external table in the hive is when you want to create the table from a file either CSV or text

Circumjacent

Answer 12

INTERNAL : Table is created First and Data is loaded later

EXTERNAL : Data is present and Table is created on top of it.

Knurl

Not necessarily, you can create an external table and insert data later in it – Bead

Answer 13

Both Internal and External tables are owned by HIVE. The only difference being the ownership of data. The commands for creating both tables are shown below. Only an additional EXTERNAL keyword comes in case of external table creation. Both tables can be created/deleted/modified using SQL Statements.

In case of Internal Tables, both the table and the data contained in the tables are managed by HIVE. That is, we can add/delete/modify any data using HIVE. When we DROP the table, along with the table, the data will also get deleted.

Eg: CREATE TABLE tweets (text STRING, words INT, length INT) ROW FORMAT DELIMITED FIELDS TERMINATED BY ',' STORED AS TEXTFILE;

In case of External Tables, only the table is managed by HIVE. The data present in these tables can be from any storage locations like HDFS. We cant add/delete/modify the data in these tables. We can only use the data in these tables using SELECT statements. When we DROP the table, only the table gets deleted and not the data contained in it. This is why its said that only meta-data gets deleted. When we create EXTERNAL tables, we need to mention the location of the data.

Eg: CREATE EXTERNAL TABLE tweets (text STRING, words INT, length INT) ROW FORMAT DELIMITED FIELDS TERMINATED BY ',' STORED AS TEXTFILE LOCATION '/user/hive/warehouse/tweets';

Conium

You can use INSERT OVERWRITE TABLE to insert records into an external table – Cicisbeo

Answer 14

hive stores only the meta data in metastore and original data in out side of hive when we use external table we can give location' ' by these our original data wont effect when we drop the table

Catchfly

Answer 15

When there is data already in HDFS, an external Hive table can be created to describe the data. It is called EXTERNAL because the data in the external table is specified in the LOCATION properties instead of the default warehouse directory.

When keeping data in the internal tables, Hive fully manages the life cycle of the table and data. This means the data is removed once the internal table is dropped. If the external table is dropped, the table metadata is deleted but the data is kept. Most of the time, an external table is preferred to avoid deleting data along with tables by mistake.

Tragus

Answer 16

For managed tables, Hive controls the lifecycle of their data. Hive stores the data for managed tables in a sub-directory under the directory defined by hive.metastore.warehouse.dir by default.

When we drop a managed table, Hive deletes the data in the table.But managed tables are less convenient for sharing with other tools. For example, lets say we have data that is created and used primarily by Pig , but we want to run some queries against it, but not give Hive ownership of the data.

At that time, external table is defined that points to that data, but doesn’t take ownership of it.

Selmore

Answer 17

In Hive We can also create an external table. It tells Hive to refer to the data that is at an existing location outside the warehouse directory. Dropping External tables will delete metadata but not the data.

Messiaen

Answer 18

I would like to add that

1. Internal tables are used when the data needs to be updated or some rows need to be deleted because ACID properties can be supported on the Internal tables but ACID properties cannot be supported on the external tables.
2. Please ensure that there is a backup of the data in the Internal table because if a internal table is dropped then the data will also be lost.

Seedman

Answer 19

In simple words, there are two things:

Hive can manage things in warehouse i.e. it will not delete data out of warehouse. When we delete table:

1) For internal tables the data is managed internally in warehouse. So will be deleted.

2) For external tables the data is managed eternal from warehouse. So can't be deleted and clients other then hive can also use it.

Namara