I have a question regarding creating tables in Hadoop.
I create external table the following way:
CREATE EXTERNAL HADOOP TABLE SCHEMA.TABLENAME (
ID BIGINT NOT NULL,
CODE INTEGER,
"VALUE" DOUBLE
STORED AS ORC
TBLPROPERTIES ('bigsql.table.io.doAs'='false',
'bucketing_version'='2',
'orc.compress'='ZLIB',
'orc.create.index'='true')
After I created this table I run Jenkins job (with sqoop process) which loads 70.000.000 records to this table.
Then I needed to remove this table, so I run:
DROP TABLE SCHEMA.TABLENAME
Later on I want to create a table with the same name as the previous one, but I need it to be empty. I make the same query as earlier, I do:
CREATE EXTERNAL HADOOP TABLE SCHEMA.TABLENAME (
ID BIGINT NOT NULL,
CODE INTEGER,
"VALUE" DOUBLE
STORED AS ORC
TBLPROPERTIES ('bigsql.table.io.doAs'='false',
'bucketing_version'='2',
'orc.compress'='ZLIB',
'orc.create.index'='true')
But when I create table this way, it has 70.000.000 records inside it again, although I didn't run any job to populate it.
This is why I have two questions:
When I drop and create table with old name, then is it recovering records from the old table?
How can I drop (or truncate) table in bigsql/hive so that I have an empty table with the old name.
I am using bigsql and hive.
Dropping an external table doesn't remove the stored data, only the metadata from the Hive Metastore.
Refer Managed vs External Tables
Key points...
Use external tables when files are already present or in remote locations
files should remain even if the table is dropped
Create a managed table (remove EXTERNAL from your query), if you want to be able to DROP and/or TRUNCATE.
Or have your Jenkins job run hadoop fs -rm -skipTrash before the import.
Related
I run a query on Databricks:
DROP TABLE IF EXISTS dublicates_hotels;
CREATE TABLE IF NOT EXISTS dublicates_hotels
...
I'm trying to understand why I receive the following error:
Error in SQL statement: AnalysisException: Cannot create table ('default.dublicates_hotels'). The associated location ('dbfs:/user/hive/warehouse/dublicates_hotels') is not empty but it's not a Delta table
I already found a way how to solve it (by removing it manually):
dbutils.fs.rm('.../dublicates_hotels',recurse=True)
But I can't understand why it's still keeping the table?
Even though that I created a new cluster (terminated the previous one) and I'm running this query with a new cluster attached.
Anyone can help me to understand that?
I also faced a similar problem, then tried the command line CREATE OR REPLACE TABLE and it solved my problem.
DROP TABLE & CREATE TABLE work with entries in the Metastore that is some kind of database that keeps the metadata about databases and tables. There could be the situation when entries in metastore don't exist so DROP TABLE IF EXISTS doesn't do anything. But when CREATE TABLE is executed, then it additionally check for location on DBFS, and fails if directory exists (maybe with data). This directory could be left from some previous experiments, when data were written without using the metastore.
if the table created with LOCATION specified - this means the table is EXTERNAL, so when you drop it - you drop only hive metadata for that table, directory contents remains as it is. You can restore the table by CREATE TABLE if you specify the same LOCATION (Delta keeps table structure along with it's data in the directory).
if LOCATION wasn't specified while table creation - it's a MANAGED table, DROP will destroy metadata and directory contents
I am creating a managed table via Impala as follows:
CREATE TABLE IF NOT EXISTS table_name
STORED AS parquet
TBLPROPERTIES ('transactional'='false', 'insert_only'='false')
AS ...
This should result in a managed table which does not support HIVE-ACID.
However, when I run the command I still end up with an external table.
Why is this?
I found out in the Cloudera documentation that neglecting the EXTERNAL-keyword when creating the table does not mean that the table definetly will be managed:
When you use EXTERNAL keyword in the CREATE TABLE statement, HMS stores the table as an external table. When you omit the EXTERNAL keyword and create a managed table, or ingest a managed table, HMS might translate the table into an external table or the table creation can fail, depending on the table properties.
Thus, setting transactional=false and insert_only=false leads to an External Table in the interpretation of the Hive Metastore.
Interestingly, only setting TBLPROPERTIES ('transactional'='false') is completly ignored and will still result in a managed table having transactional=true).
I created 2 external tables Hive. In first table specified data location with create statement. In second table loaded data after creating it.
I can see data file created for second table in /hive/warehouse/ directory. Then I set "external.table.purge"="true" for both tables. And DROP both tables. But data files of both tables remains as is.
What is the behaviour of 'external.table.purge'='true'. Shouldn't it delete data files as well on issuing Drop command?
If Hive does not take any ownership over data files of external table, why is there even an option as 'external.table.purge'='true'.
I read in one of the threads, where someone mentioned it is possible to delete data as well for external tables by ALTER TABLE ... SET TBLPROPERTIES('external.table.purge'='true'), but unable to find that post again.
You can not drop the data in external table but you can do it for internal(managed) tables. So convert the table to internal and then drop it.
First change eternal property to false.
hive> ALTER TABLE nyse_external SET TBLPROPERTIES('EXTERNAL'='False');
and then you can easily drop it.
hive> drop table nyse_external;
TBLPROPERTIES ("external.table.purge"="true") should work for hive version 4.x+.
Answer to point 1:
Table property "external.table.purge", which if true (and if the table is an external table), will let Hive know to delete the table data when the table is dropped. This feature is introduced in this apache jira.
https://issues.apache.org/jira/browse/HIVE-19981 .
For reference on how to set the property take a look at this example,
https://docs.cloudera.com/runtime/7.2.7/using-hiveql/topics/hive_drop_external_table_data.html
When I am using DROP TABLE IF EXISTS <Table Name> in hive, it is not freeing the memory. The files are created as 0000_n.bz2 and they are still on disk.
I have two questions here:
1) Will these files keep on growing for each and every insert?
2) Is there any DROP equivalent to remove the files as well on the disk?
Couple of things you can do:
Check if the table is an external table and in that case you need to drop the files manually on HDFS as dropping tables won't drop the files:
hadoop fs -rm /HDFS_location/filename
Secondly check if you are in the right database. You need to issue use database command before dropping the tables. The database should be same as the one in which tables were created.
There are two types of tables in hive.
Hive managed table: If you drop a hive managed table the data in HDFS are automatically deleted.
External Table: If you drop an external table, hive doesnt delete the underlying data.
I believe yours is an external table.
Drop table if exists table_name purge;
This command will also remove data files from trash folder and cannot be recovered after table drop
I see options that allow to add new columns in Hive [source]
However, I have EXTERNAL Table which is mapped with HBase with SERDEPROPERTIES, TBLPROPERTIES and STORED BY 'org.apache.hadoop.hive.hbase.HBaseStorageHandler'.
Is it possible to add/alter columns after External table with HBase is setup?
Do I just SERDEPROPERTIES for the new columns or do we need to re-do the whole table?
When you try to use ALTER TABLE xx ADD COLUMS( xx string); you get the following error.
SemanticException [Error 10134]: ALTER TABLE cannot be used for a non-native table hbase_cdr2
So looking at this, it seems there is no way to update existing table by adding new columns. But you can drop the hive table, create a new table with required columns. Since it is an external table you only update the metadata by doing that.