Category Archives: Hive

Hive and integer overflows

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I was playing around in Hive (3, from HDP) with big integers when I noticed something weird:

select 9223372036854775807 * 9223372036854775807;
-- 1

select 9223372036854775807 + 1;
-- -9223372036854775808

It turns out that Hive silently overflows integers. This comes from java, which does the same.

It’s lucky I noticed, it could have been very painful for me down the line. The workaround is to use big decimal. As the doc says:

Integral literals larger than BIGINT must be handled with Decimal(38,0). The Postfix BD is required.

For instance:

select 9223372036854775807BD * 9223372036854775807BD;
-- 85070591730234615847396907784232501249

select 9223372036854775807BD + 1;
-- 9223372036854775808

But it is still somewhat weird. Overflows with big decimals won’t error out but will return null:

select 9223372036854775807BD
  * 9223372036854775807BD
  * 9223372036854775807BD
;
-- NULL

Furthermore, if the precision is not 0 some behaviours are not consistent:

create temporary table dec_precision(
  d decimal(38, 18)
);
insert into dec_precision values
    (98765432109876543210.12345)
  , (98765432109876543210.12345)
;

select sum(d) from dec_precision;
-- NULL (but why?)
select sum(98765432109876543210.12345BD) from dec_precision;
-- 197530864219753086420.24690 (as expected)
select 98765432109876543210.12345BD + 98765432109876543210.12345BD;
-- 197530864219753086420.24690 (as expected)

Conversely, Mysql

select 9223372036854775807 * 9223372036854775807;
-- ERROR 1690 (22003): BIGINT value is out of range in '9223372036854775807 * 9223372036854775807'

or Postgres

select 2147483647 * 2147483647;
ERROR:  integer out of range

are a lot safer and friendlier in that regard.

Easy test data with Hive

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Testing a query on a small dataset, especially if you need to carefully check your joins is usually made by creating a few temporary tables with hand-crafted data. This is a true and tested method, but it has a few disadvantages:

  • Requires some work if you need to change your data,
  • If the table is not temporary you need to not forget to drop it,
  • If your table is temporary it needs to be recreated after a reconnection,
  • If you don’t save the initialisation statements your test data is gone,
  • Especially with Hive, handling small tables has a lot of overhead.

It all works, but there is a nicer alternative: CTE + UDTF. Expanded, it means Common Table Expression with User Defined Table-generating Function.

Without further ado, here is an example, with the usual employees and departement:

with employee as(
  select inline(array(
      struct('Alice', '2017-03-04', 1)
    , struct('Bob', '2017-04-12', 1)
    , struct('Carol', '2018-12-24', 2)
  ))  as (name, start_date, dpt_id)
)
, department as (
  select inline(array(
      struct('IT', 1)
    , struct('Finance', 2)
  ))  as (name, id)
)
select
    e.name
  , e.start_date
  , d.name
from
  employee e
join
  department d
on
  e.dpt_id=d.id
;

And the result:

+---------+---------------+----------+
| e.name  | e.start_date  |  d.name  |
+---------+---------------+----------+
| Alice   | 2017-03-04    | IT       |
| Bob     | 2017-04-12    | IT       |
| Carol   | 2018-12-24    | Finance  |
+---------+---------------+----------+

So, what do we have here?

I define 2 common table expressions (with .. as () statement), which is a sort of run-time table. They can be used in any following CTE or queries. This table is defined by just giving the data we want in it (surrounded by inline(array(…)) as). Changing, adding, removing data is thus trivial and all is nicely bundled in one place.

Another nice thing is that these CTEs actually shadow real tables with the same name. This means that once you’re done testing, you just comment out the CTE definitions and the query will run with real data. This has the added benefit that you can always keep your test data with your actual query. You just need to uncomment the CTEs to use them.

Many other RDBMs (Mysql, Postgres, Oracle…) have CTEs. The UDTF (inline function) is less common, unfortunately.

Reaching Hive from pyspark on HDP3

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There is a lot to find about talking to hive from Spark on the net. Sadly most of it refers to Spark before version 2 or are not valid for hdp3. You need to use the Hive Warehouse Connector, bundled in HDP3.

This is an example of a minimalistic connection from pyspark to hive on hdp3.

from pyspark.sql import SparkSession
from pyspark.conf import SparkConf

# Yes, llap even if you do not use it.
from pyspark_llap import HiveWarehouseSession

settings = [
    ('spark.sql.hive.hiveserver2.jdbc.url',
     'jdbc:hive2://{your_hiverserver2_url:port}/default'),
]

conf = SparkConf().setAppName("Pyspark and Hive!").setAll(settings)
# Spark 2: use SparkSession instead of SparkContext.
spark = (
    SparkSession
    .builder
    .config(conf=conf)
    .master('yarn')
    # There is no HiveContext anymore either.
    .enableHiveSupport()
    .getOrCreate()
)

# This is mandatory. Just using spark.sql will not be enough.
hive = HiveWarehouseSession.session(spark).build()

hive.showDatabases().show()
hive.execute("select 2 group by 1 order by 1").show()
spark.stop()

You then can run this with the following command:

HDP_VERSION=3.0.1.0-187 \
PYSPARK_PYTHON=python3 \
HADOOP_USER_NAME=hive \
SPARK_HOME=/usr/hdp/current/spark2-client \
spark-submit \
--jars /usr/hdp/current/hive_warehouse_connector/hive-warehouse-connector-assembly-1.0.0.3.0.1.0-187.jar \
--py-files /usr/hdp/current/hive_warehouse_connector/pyspark_hwc-1.0.0.3.0.1.0-187.zip \
{your_python_script.py}

Note:

  • HDP_VERSION is needed when you use python 3. If not set, HDP uses a script (/usr/bin/hdp-select) which is python 2 only (although fixing it is trivial).
  • PYSPARK_PYTHON is optional, it will default to just python otherwise (which might or might not be python 3 on your server)
  • without HADOOP_USER_NAME the script will run as your current user. Alternatively, you could sudo first.
  • without SPARK_HOME some jars would not be found and you would end up with an error like py4j.protocol.Py4JJavaError: An error occurred while calling None.org.apache.spark.api.java.JavaSparkContext.
    : java.lang.NoClassDefFoundError: com/sun/jersey/api/client/config/ClientConfig
  • –jars and –py-files as you can see, there is the hdp version in file names. Make sure you are using the proper one for you.
  • there is no –master option, this is handled in the script while building the SparkSession.

There is some doc from Hortonworks you can follow to go further: Integrating Apache Hive with Spark and BI.

Just before I posted this article, a new write-up appeared on Hortonworks.com to describe some use cases for the Hive-Warehouse-Connector.

Oops, I dropped my Hive Metastore database

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The Hive Metastore (HMS) is backed up by a database (MySQL for us), which stores information about HDFS files, stats and more. Without this database, you have no Hive.

A few days ago, a DROP has been issued on the wrong server. Bye Bye metastore. What do you do then?

First things first, I looked at the backups. We had one from 3 hours before, so still quite recent. After it was restored I noticed that Hive worked, but not all data was present visible.

For context, our tables are ORC and transactional. As HDFS files cannot be updated, the way for ORC (and other DBs I know of) to manage transactions is to have a base directory as well as some deltas holding new changes. Reading such a table thus means reading the base directory and all the deltas to apply potential updates. As reading the deltas can become expensive, they are eventually compacted, in 2 possible ways. Quite often the deltas are squashed together (minor compaction for ORC) and once in a while, the base directory is fully rewritten to apply all the deltas (major compaction for ORC).

In my case, between the backup and the restore some new data had been added (new delta directories appeared) but no compaction happened.

This means that the HMS was completely ignoring all the new delta files, but was happily using all the files it knew about. I should add that I have a way to replay data of the last hours/days if needed. Once this was understood and confirmed, the fix was easy:

 

  1. Run a major compaction on all tables
    • All tables ended up consisting of one base directory (used and known by HMS) and a few delta directories ignored by HMS.
  2. Delete all remaining deltas
    • They were not used anyway.
  3. Replay my data
    • Hive was up to date again.

Of course, my tables are partitioned and compaction happens per partition so there was a bit of bash-fu to explicitly compact all partitions, then double check that all remaining deltas were created after the backup. Nothing too complex.

This worked because no compactions ran between restore and backup. A compaction would completely change the files on disk, and the HMS metadata would thus be completely out of sync with the actual files. I have no idea how I would have recovered that one.

On a side note, Hive hanged a few times with this error message in the log:

insert into “NOTIFICATION_LOG
” (“NL_ID”, “EVENT_ID”, “EVENT_TIME”, “EVENT_TYPE”, “DB_NAME”, “TBL_NAME”, “MESSAGE”, “MESSAGE_FORMAT”) values(774946,774869,1543829236,’OPEN_TXN’,’null’,’ ‘,'{“txnIds”:null,”timestamp”:1543829236,”fromTxnId”
:54788,”toTxnId”:54788,”server”:”thrift://x.x.x.x:9083,thrift://y.y.y.y:9083″,”servicePrincipal”:”hive/_HOST@EXAMPLE.COM”}’,’json-0.2′)

metastore.RetryingHMSHandler (RetryingHMSHandler.java:invokeInternal(201)) – MetaException(message:Unable to execute direct SQL java.sql.SQLInte
grityConstraintViolationException: Duplicate entry ‘774946’ for key ‘PRIMARY’

The fix was quite easy. Log into the metastore DB, remove the offending row from NOTIFICATION_LOG, and update the sequence (NOTIFICICATION_LOG_SEQUENCE table) to the maximum value of NL_ID in NOTIFICATION_LOG + 1. The NOTIFCATION_LOG table has something to do with compactions, and as you compacted all tables anyway old values do not matter much.

What is the takeaway? Compactions are run dynamically by Hive. It would be smart to keep an eye on them, and when some happened to run a backup just after. At least you would still be able to partially recover consistent recent enough data.

Newlines in Hive

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I have a nice Hive warehouse, running data exports for some clients. All was working smoothly when a coworker came to me because a client complained that there were NULLs in an export, whereas they should all be sanitised via a lot of coalesce.

The reason ended up being newlines which I thought were properly handled, but as it turned out were not in all cases. This post explains what the issues are and how to fix them.

This is an example of what happened (simplified query to show the problem). A json string with newlines, exploded with a lateral view. Newlines are escaped so all should have worked.

SET system:disable.quoting.for.sv=false;
with things as (
    select
        42 as id
      , '{   "item": "cheese"
           , "good": "camembert\\ncomté"
           , "tasty": "yes"
         }' as custom_fields
)
select
    c.id
  , c.custom_fields
  , cv.good
  , cv.item
  , cv.tasty
from
    (select * from things order by id) c
lateral view
    json_tuple(c.custom_fields, "good", "item", "tasty") cv
    as good, item, tasty
;

If you run this code you might (depending on your setup) see a nice \n in custom_fields but an actual line breaks in good which is not what you want.

Note that if you replace the from clause by just from things c the issue will not appear.

The problem is that in my example an intermediary map step is executed (because of the order by). Data in this step was stored by default as textfile, where the row delimiter is, you can guess, \n.

With the simplified from clause, there is no intermediary map step and the problem does not happen.

What we would like is to have the intermediary data stored as some sort of binary format, where \n is not a separator. Luckily, there’s a setting for that: hive.query.result.fileformat. The default (up to Hive 2.0) was TextFile, which as you can guess will use newlines as row separator. From Hive 2.1 onward, the default became SequenceFile which is a binary format, compressible, which does not have the newlines issue.

You can either set this setting globally in hive-site, or add it before your query with set hive.query.result.fileformat=SequenceFile;.

With Hive 2, this fixes the newlines, but the \n completely disappear from the output. This is better than previously but not ideal.
With Hive 3, if you are using beeline, you can add the command line option --escapeCRLF=true which will give you exactly the output you want.

Anatomy of a Merge statement in Hive

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When a merge statement is issued, it is actually reparsed in a bunch of inserts. This shows interesting properties which can help you better understand the performance of your statement.

The test setup is easy: one source table, one destination table. The destination needs to be ACID and thus bucketed (this is pre-Hive 3).

create table tmp.tstmerge
(b int, v string)
partitioned by (id int)
clustered by (b) into 1 buckets
stored as orc
TBLPROPERTIES ("transactional"="true");

create table tmp.srcmerge
stored as orc
as
select 1 as id, 'foo' as v;

The merge statement is trivial as well:

merge into tmp.tstmerge dst
using tmp.srcmerge src
on src.id=dst.id
when matched and src.id = 0 then delete
when matched then update set
     v=concat(dst.v, src.v)
when not matched then insert values (
    src.b, src.v, src.id
);

What becomes interesting is that you can see in hiveserver2.log a line like:

Going to reparse YOUR_STATEMENT as ANOTHER_STATEMENT

For the given merge, here is the actual ANOTHER_STATEMENT:

FROM
    tmp.tstmerge dst
RIGHT OUTER JOIN
    tmp.srcmerge src
ON
    src.id=dst.id
INSERT INTO tmp.tstmerge partition (id)    -- delete clause
    select
        dst.ROW__ID , dst.id
    WHERE
        src.id=dst.id AND src.id = 0
    sort by
        dst.ROW__ID
INSERT INTO tmp.tstmerge partition (id)    -- update clause
    select
        dst.ROW__ID, dst.b, concat(dst.v, src.v), dst.id
    WHERE
        src.id=dst.id AND NOT(src.id = 0)
    sort by
        dst.ROW__ID
INSERT INTO tmp.tstmerge partition (id)    -- insert clause
    select
        src.b, src.v, src.id
    WHERE
        dst.id IS NULL
INSERT INTO merge_tmp_table
    SELECT
        cardinality_violation(dst.ROW__ID, dst.id)
    WHERE
        src.id=dst.id
    GROUP BY
        dst.ROW__ID, dst.id
    HAVING
        count(*) > 1

What do we have here?

  1. This becomes a multi-insert statement with 3 different selections for the update, insert and delete clauses. The multi insert is a hive extension.
  2. ROW__ID is a hidden column for acid tables, containing a map: 
    select row__id from tmp.tstmerge;
+----------------------------------------------+--+
| row__id |
+----------------------------------------------+--+
| {"transactionid":44,"bucketid":0,"rowid":0} |
+----------------------------------------------+--+
  3. To update a row, you just need to change the columns of a row identified by its ROW__ID. Deleting a row is equivalent to nullifying all columns of a ROW__ID. This works because all clauses are insert in the ORC delta files anyway.
  4. cardinality_violiation is a UDF which will exception out if more than one row has the same set of ROW__ID and join condition. This is because the SQL syntax says that there cannot be more than 1 row in the source matching the destination. It will be executed (and thus exception out) only if such a row exists. On a side note, if you prevent cardinality checking (set hive.merge.cardinality.check=false;) then this leg of the multi insert does not exist.
  5. Rows are sort by ROW__ID. Note first that sort by will sort per reducer, whereas order by would sort across all reducers. Order by would be prohibitively expensive. The reason for the sorting is that when delta files are read they can be directly merged on read.

Practically this means that you cannot order data in ORC ACID tables (which is a shame as it is the one thing to do performance-wise when using ORC). Furthermore, any ordering in the src statement, if not meant to speed the join up, will be useless.

The cost of ACID with ORC table

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ACID introduction

ACID transactions (update, merge) in Hive are awesome. The merge statement especially is incredibly useful.

Of course, not all table are ACID. You need to use ORC and have the table marked as ACID but those are easy steps:

create table something (id bigint) stored as orc tblproperties("transactional"="true")

Of course, in hdfs you cannot change a file once it is created. The standard way (not Hadoop specific) to handle changing immutable files is to have deltas. Each table will consist of a few directories:

  • the base directory: the data at creation time,
  • one or more delta directories: contains updated rows.

Every hive.compactor.check.interval seconds a compaction will happen (or at least the compactor will check if a compaction must happen). The compactor will compact the deltas and base directory in a new base directory, which will consist of a one new base directory with all the deltas applied to the original base directory.

The reason is that when you read an ACID table with many deltas, there is a lot more to read than for only a base directory as hive has to go through each and every delta. This has IOs and CPU costs, which are removed after compaction.

Naive ACID use

Every day I build a summary table gathering all data that changed in the last 24h as well as some related data. Many events are aggregated together. Think for instance about sending an email: I would get send data, open data maybe click data, bounce and a few others. I started building following the temporal flow:


create table summary (id bigint, number_sent bigint, number_open bigint...)stored as orc tblproperties("transactional"="true");

insert into summary select .... from sent;

merge into summary select ... from open;

merge into summary select ... from click;

...

Overall a few billions rows will be read. The final summary table will have about 100 millions rows.

What is interesting here is that I am inserting the biggest data first. This is the table summing up reads and writes per event while building the whole summary, which ran for about 4 hours:

Event Bytes read (GB) Bytes written (GB)
Total 516.5 104.1
Sent 16.2 87.1
Open 88.8 14.2
Click 101.5 1.7
Conversion 102.9 0.01
Bounce 103 1
Spam 104 0.11

Seeing 500GB read scared me a little, so instead of following the naive temporal flow, I started with the smallest event first to finish up with the biggest:

Event Bytes read (GB) Bytes written (GB)
Total 31.5 99.1
Conversion 0 0
Spam 0 0
Click 0.3 1.5
Bounce 1.7 1
Open 4.4 13.3
Sent 25.1 83.4

That’s much better already! The total number of bytes written does not change much (quite logical I suppose as the final data is the same) but the number of bytes read is only 6% of the original! Furthermore, it ran in 2h40 instead of 4 hours.

I added one last step. This summary data was written at user level. I actually needed to do one extra aggregation but I was worried about joining against the user table at every step, as the user table is actually quite big and joins are expensive. But well, I experimented, doing the aggregation at each step instead of  doing one big aggregation at the end:

Event Bytes read (GB) Bytes written (GB)
Total 20.5 8.6
Conversion 0.2 0
Spam 1.2 0
Click 1.4 0.2
Bounce 1.5 0.2
Open 3.5 1.7
Sent 12.7 6.4

Total run time: 1.5 hours!

TL;DR

When using ACID deltas are expensive. When using HDFS writes are expensive. Order your processing to have a little of those as possible.

Find a timezone offset in pure SQL in hive

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Timezones are a pain. This is not new and every time you deviate from UTC this will bite you. That said sometimes you have to deviate from UTC, especially for the final display of a date if you want to show it in the local timezone from the reader. In that case, adding an offset to be explicit will save some questions and uncertainty down the line.

There is no function get_offset_from_tz() in Hive, sadly. Using reflect() does not work either as the method call is to complex for reflect. Writing a UDF would be possible but feels overkill.

The solution I give here works in Hive and should probably work in all SQL variants as well apart from the variables.

The algorithm to find the offset is easy:

  • get the time in UTC,
  • get the same in another timezone,
  • subtract one from the other to get the offset,
  • format the offset in a standard way.

The main issue is that you cannot assign results to variables in SQL, meaning that many computations need to be duplicated. They will be optimised away, of course, but they make for an ugly code.

In hive, luckily, you can use variables. They cannot store results but are used as-is, a bit like macros, where the variable name is just replaced by its content which can be some piece of code.

This sets up the date to find the offset for as well as a few TZ for test.

-- Date to display. If you use this from a table you can
-- put here the column that would be used, eg. t.logdate.
set hivevar:D='2018-06-01 01:02:02';

-- A few tests:
-- positive offset +02:00 (in summer)
set hivevar:DISPLAY_TZ='Europe/Amsterdam';

-- negative offset -04:00 (in summer)
set hivevar:DISPLAY_TZ='America/New_York';

-- 0 offset
set hivevar:DISPLAY_TZ='UTC';

-- Non integer offset: +09:30
set hivevar:DISPLAY_TZ='Australia/Adelaide';

Those are the macros


-- Date displayed in the right TZ
set hivevar:dateintz=DATE_FORMAT(FROM_UTC_TIMESTAMP(${D}, ${DISPLAY_TZ}),"yyyy-MM-dd HH:mm:ss");
-- Offset in interval type
set hivevar:delta=cast(${dateintz} as timestamp) - cast(${D} as timestamp);

And the code itself, tiny and readable once variables are used:

select
  concat(
    -- date in TZ
    ${dateintz}

    -- sign
    , if(${delta} < interval '0' minute, '-', '+')

    -- hour
    , lpad(abs(hour(${delta})), 2, 0)

    , ':'

    -- minute
    ,lpad(minute(${delta}), 2, 0)
) as dtwithoffset
;

et voilà.

Hive, map, NULL and NPE

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Checking for null values in a map column in Hive (1.2.1, Hortonworks) interestingly returns a null pointer exception:

create table npe (m map);
select count(*) from npe where m is null;
Error: Error while compiling statement: FAILED: NullPointerException null (state=42000,code=40000)

The error happens at parsing time when Hive tries to estimate data size. From hiveserver2.log:

Caused by: java.lang.NullPointerException
at org.apache.hadoop.hive.ql.stats.StatsUtils.getSizeOfMap(StatsUtils.java:1096)

Interestingly, getting not null is fine:

select count(*) from npe where m is not null; -- returns 0

If you think like me, you will think ‘haha! not not null should work!’

select count(*) from npe where not m is not null; -- does not work

If you are smarter than me, you will have guessed before trying that Hive optimises the double negation away, and gives another NPE.

But going in this direction, we can still trick Hive by casting the boolean to int:

select count(*) from npe where int(m is not null)=0; -- works

This happens either without data either when there are real NULLs in the table. By real NULL I mean that a SELECT would show NULL, which happens only in the case where you add a column to an existing table. Indeed, you cannot yourself insert NULL into a complex column:

with a as (select null) insert into npe select * from a;
Error: Error while compiling statement: FAILED: SemanticException [Error 10044]: Line 1:36 Cannot insert into target table because column number/types are different 'npe': Cannot convert column 0 from void to map. (state=42000,code=10044)

You have to create an empty map object:

with a as (select map(cast(null as bigint), cast(null as bigint))) insert into npe select * from a;

Then, of course, the empty map object is not (a real) NULL and if you want to look for null you have to fudge a bit, looking at the size of the map for instance:

select m, size(m), isnull(m) from npe;

+----+-----+--------+--+
| m  | _c1 | _c2    |
+----+-----+--------+--+
| {} |  0  | false  |
+----+-----+--------+--+

Hive self merge

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I had a (ORC) table with duplicated rows, which I wanted to remove. The query is quite simple:

merge into click as dst using (
    select
      -- For all unique clicks...
        client_name
      , contact_id
      , ts_utc
      -- ... find the duplicates (cnt>1 in having) ...
      , count(*) cnt
      -- ... remember the first one loaded ...
      , min(load_ts) as first_load
      from
        click
      group by
        1, 2, 3
      having cnt > 1
)
as src
-- ... once the first occurrence of the duplicates
-- is found find all the duplicates ...
on
        dst.client_name=src.client_name
    and dst.contact_id=src.contact_id
    and dst.ts_utc=src.ts_utc
-- ... and if it is not the first one loaded ...
when matched and src.first_load != dst.load_ts
-- .. delete it.
then delete
;

Trivial, right? Well it looks like you cannot do such a ‘self merge’ in hive. I ended up with this error:

java.lang.InterruptedException
 at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.reportInterruptAfterWait(AbstractQueuedSynchronizer.java:2014)
 at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2048)
 at org.apache.tez.runtime.InputReadyTracker$InputReadyMonitor.awaitCondition(InputReadyTracker.java:120)
 at org.apache.tez.runtime.InputReadyTracker.waitForAllInputsReady(InputReadyTracker.java:90)
 at org.apache.tez.runtime.api.impl.TezProcessorContextImpl.waitForAllInputsReady(TezProcessorContextImpl.java:116)
 [...]

The solution, once understood that a self merge is not allowed, is of course obvious: use a temporary table. Splitting my merge statement in 2 did the trick.

create temporary table clickdups stored as orc as select
        client_name
      , contact_id
      , ts_utc
      , count(*) cnt
      , min(load_ts) as first_load
      from
        click
      group by
        1, 2, 3
      having cnt > 1
;

merge into click as dst using clickdups
as src
on
        dst.client_name=src.client_name
    and dst.contact_id=src.contact_id
    and dst.ts_utc=src.ts_utc
when matched and src.first_load != dst.load_ts
then delete
;

On a side note I needed to tweak a lot the self-merge to prevent out of memory error. Those did not happen at all using the 2 steps solution.