When I load a (csv)-file to a hive table I can load without overwriting, thus adding the new file to the table.
Internally the file is just copied to the correct folder in HDFS
(e.g. user/warehouse/dbname/tablName/datafile1.csv). And probably some metadata is updated.
After a few loads I want to remove the contents of a specific file from the table.
I am sure I cannot simply delete the file because of the metadata that needs to be adjusted as well. There must be some kind of build-in function for this.
How do I do that?
Why do you need that?I mean Hive was developed to serve as a warehouse where you put lots n lots n lots of data and not to delete data every now and then. Such a need seems to be a poorly thought out schema or a poor use of Hive, at least to me.
And if you really have these kind of needs why don't you create partitioned tables? If you need to delete some specific data just delete that particular partition using either TRUNCATE or ALTER.
TRUNCATE TABLE table_name [PARTITION partition_spec];
ALTER TABLE table_name DROP [IF EXISTS] PARTITION partition_spec, PARTITION partition_spec,...
if this feature is needed more than just once in a while you can use MapR's distribution while allows this kind of operations with no problem (even via NFS). otherwise, if you don't have partition I think you'll have to create and new table using CTAS filterring the data in the bad file or just copy the good files back to os with "hadoop fs -copyToLocal" and move them back to hdfs into new table
Related
Is it even possible to add a partition to an existing table in Athena that currently is without partitions? If so, please also write syntax for doing so in the answer.
For example:
ALTER TABLE table1 ADD PARTITION (ourDateStringCol = '2021-01-01')
The above command will give the following error:
FAILED: SemanticException table is not partitioned but partition spec exists
Note: I have done a web-search, and variants exist for SQL server, or adding a partition to an already partitioned table. However, I personally could not find a case where one could successfully add a partition to an existing non-partitioned table.
This is extremely similar to:
SemanticException adding partiton Hive table
However, the answer given there requires re-creating the table.
I want to do so without re-creating the table.
Partitions in Athena are based on folder structure in S3. Unlike standard RDBMS that are loading the data into their disks or memory, Athena is based on scanning data in S3. This is how you enjoy the scale and low cost of the service.
What it means is that you have to have your data in different folders in a meaningful structure such as year=2019, year=2020, and make sure that the data for each year is all and only in that folder.
The simple solution is to run a CREATE TABLE AS SELECT (CTAS) query that will copy the data and create a new table that can be optimized for your analytical queries. You can choose the table format (Parquet, for example), the compression (SNAPPY, for example), and also the partition schema (per year, for example).
This is my first week with Hive and HDFS, so please bear with me.
Almost all the ways I saw so far to merge multiple ORC files suggest using ALTER TABLE with CONCATENATE command.
But I need to merge multiple ORC files of the same table without having to ALTER the table. Another option is to create a copy of the existing table and then use ALTER TABLE on that so that my original table remains unchanged. But I can't do that as well because space and data redundancy reasons.
The thing I'm trying to achieve (ideally) is: I need to transport these ORCs as one file per table into a cloud environment. So, is there a way that I can merge the ORCs on-the-go during the transfer process into cloud? Can this be achieved with/without Hive, maybe directly in HDFS?
Two possible methods other than ALTER TABLE CONCATENATE:
Try to configure merge task, see details here: https://stackoverflow.com/a/45266244/2700344
Alternatively you can force single reducer. This method is quite applicable for not too big files. You can overwrite the same table with ORDER BY, this will force single reducer on the last ORDER BY stage. This will work slow or even fail with big files because all the data will be passed through single reducer:
INSERT OVERWRITE TABLE
SELECT * FROM TABLE
ORDER BY some_col; --this will force single reducer
As a side effect you will get better packed ORC file with efficient index on columns listed in order by.
Is there any number of partitions we would expect this command
MSCK REPAIR TABLE tablename;
to fail on?
I have a system that currently has over 27k partitions and the schema changes for the Athena table we drop the table, recreate the table with say the new column(s) tacked to the end and then run
MSCK REPAIR TABLE tablename;
We had no luck with this command doing any work what so every after we let it run for 5 hours. Not a single partition was added. Wondering if anyone has information about a partition limit we may have hit but can't find documented anywhere.
MSCK REPAIR TABLE is an extremely inefficient command. I really wish the documentation didn't encourage people to use it.
What to do instead depends on a number of things that are unique to your situation.
In the general case I would recommend writing a script that performed S3 listings and constructed a list of partitions with their locations, and used the Glue API BatchCreatePartition to add the partitions to your table.
When your S3 location contains lots of files, like it sounds yours does, I would either use S3 Inventory to avoid listing everything, or list objects with a delimiter of / so that I could list only the directory/partition structure part of the bucket and skip listing all files. 27K partitions can be listed fairly quickly if you avoid listing everything.
Glue's BatchCreatePartitions is a bit annoying to use since you have to specify all columns, the serde, and everything for each partition, but it's faster than running ALTER TABLE … ADD PARTION … and waiting for query execution to finish – and ridiculously faster than MSCK REPAIR TABLE ….
When it comes to adding new partitions to an existing table you should also never use MSCK REPAIR TABLE, for mostly the same reasons. Almost always when you add new partitions to a table you know the location of the new partitions, and ALTER TABLE … ADD PARTION … or Glue's BatchCreatePartitions can be used directly with no scripting necessary.
If the process that adds new data is separate from the process that adds new partitions, I would recommend setting up S3 notifications to an SQS queue and periodically reading the messages, aggregating the locations of new files and constructing the list of new partitions from that.
I have created a hive table with dynamic partitioning on a column. Is there a way to directly load the data from files using "LOAD DATA" statement? Or do we have to only depend on creating a non-partitioned intermediate table and load file data to it and then inserting data from this intermediate table to partitioned table as mentioned in Hive loading in partitioned table?
No, the LOAD DATA command ONLY copies the files to the destination directory. It doesn't read the records of the input file, so it CANNOT do partitioning based on record values.
If your input data is already split into multiple files based on partitions, you could directly copy the files to table location in HDFS under their partition directory manually created by you (OR just point to their current location in case of EXTERNAL table) and use the following ALTER command to ADD the partition. This way you could skip the LOAD DATA statement altogether.
ALTER TABLE <table-name>
ADD PARTITION (<...>)
No other go, if we need to insert directly, we'll need to specify partitions manually.
For dynamic partitioning, we need staging table and then insert from there.
I am moving data around within Impala, not my design, and I have lost some data. I need to copy the data from the parquet tables back to their original non-parquet tables. Originally, the developers had done this with a simple one liner in a script. Since I don't know anything about databases and especially about Impala I was hoping you could help me out. This is the one line that is used to translate to a parquet table that I need to be reversed.
impalaShell -i <ipaddr> use db INVALIDATE METADATA <text_table>;
CREATE TABLE <parquet_table> LIKE <text_table> STORED AS PARQUET TABLE;
INSERT OVERWRITE <parquet_table> SELECT * FROM <text_table>;
Thanks.
Have you tried simply doing
CREATE TABLE <text_table>
AS
SELECT *
FROM <parquet_table>
Per the Cloudera documentation, this should be possible.
NOTE: Ensure that your does not exist or use a table name that does not already exist so that you do not accidentally overwrite other data.