Asking for help, clarification, or responding to other answers. See also this answer: https://stackoverflow.com/a/51492032/2700344. , Type123Users int Hive provides SQL type querying language for the ETL purpose on top of Hadoop file system.. Hive Query language (HiveQL) provides SQL type environment in Hive to work with tables, databases, queries. Restrictions: This function cannot be used in an analytic context. ,count(distinct case when a.type != 111 then a.user else null end) as Type123User Step 3) Create 2 destination tables to simulate multi-insert. Challenges in performing accurate distinct count using Hive and Impala. 2013-12-02 2 1 1 2013-12-02 As of Hive 1.2.0 there is optimization for count(distinct) and you do not need to rewrite query. Details. It is used to query a group of records. , Type111Users int Map sends each value to the single reducer, and reducer does all the job. Ignoring the actual backtrace (which is a bug), I have seen issues with uncompressed text tables in Tez related to Hive's use of Hadoop-1 APIs. hive> select * from dest_table_a; OK 2013-12-02 2 1 1 2013-12-02 Time taken: 0.116 seconds hive> select * from dest_table_b; OK 2013-12-02 2 1 1 2013-12-02 Time taken: 0.13 seconds. 2. Create a table backed by a file in S3. Added row count validation. ( Why are there no papers about stock prediction with machine learning in leading financial journals? A related issue: Impala still does not support multiple COUNT(DISTINCT ..) expressions IMPALA-110 Insert 2 records into the table created above. , Type111Users int The DISTINCT expression may be used in Aggregate or Remove Duplicates operators. If the SELECT has 3 columns listed then SELECT DISTINCT will fetch unique row for those 3 column values only. In this article, we will look at the group by HIVE. This came up on the Hive mailing list and I'm putting it here as a reminder to try it out. Fit ellipse to a arbitrary 2D image to extract centroid, orientation, major, minor axis. Learning Mapreduce: is it the reducer that does count or the mapper. Also, to understand well, see an example below. PARTITIONED BY (dt string) Why am I getting rejection in PhD after interview? Examples: The following example queries a billion-row table to illustrate the relative performance of COUNT(DISTINCT) and NDV().It shows how COUNT(DISTINCT) gives a precise answer, but is inefficient for large-scale data where an approximate result is sufficient. Aggregate functions can also be used with the DISTINCT keyword to do aggregation on unique values. p_date string is not working properly when hive.optimize.countdistinct is true. Z query works separately 3. the whole query Z works fine in Hive (CDH 4.3) 4. Group By as the name suggests it will group the record which satisfies certain criteria. Why do SpaceX Starships look so "homemade"? @user2894829 that is rather outdated. USE CATALOG hive; CREATE TABLE dev_orders ( o_orderkey INTEGER, … FIELDS TERMINATED BY '|' Priority: Major . Exact counting in Hive hive.optimize.distinct.rewrite allows to rewrite COUNT(DISTINCT) to subquery since Hive 1.2.0 9. , Distinct_Users int Blog About Me Complex Counts in Hive. ,count(distinct case when a.type != 111 then a.user else null end) as Type123User The DISTINCT expression may be used in Aggregate or Remove Duplicates operators. ROW FORMAT DELIMITED LOCATION '/hive//Table_A'; Step 2) Scenario: Lets us say consider user tommy belong to both usertypes 111 and 123. @user2894829 I do not know about this bug. DISTINCT has a bit of an odd history in the code as an alias to group by. If it is a bug in your Hive version, create a Jira ticket. Map sends each value to the single reducer, and reducer does all the job. Table Name Pattern Explanation; Distinct Value Table ${FLAT_TABLE} _ ${kylin.dictionary.mr-hive.intermediate.table.suffix} This table is a temporary hive table for storing literal value which be extracted from flat table.. For Hive 0.11.0 through 2.1.x, set hive.groupby.orderby.position.alias to true (the default is false). There are 227,296,944rows in our test dataset. ,count(distinct a.user) as AllDist Select x from Y works separately 2. Time taken: 0.13 seconds. rev 2021.3.17.38813, Stack Overflow works best with JavaScript enabled, Where developers & technologists share private knowledge with coworkers, Programming & related technical career opportunities, Recruit tech talent & build your employer brand, Reach developers & technologists worldwide, Thanks for your detailed reply! The last question about. In legacy RDBMS like MySQL, SQL, etc., group by is one of the oldest clauses used. DISTINCT keyword is used in SELECT statement in HIVE to fetch only unique rows. Group by Query Example. In the two plans below, the aggregations part in the Output of Group By Operator of Map 1 are different. In Hive, COUNT (distinct) is a single reducer problem and goes through a massive reduce side sort. Add properties in configuration overwrite step. Is it meaningful to define the Dirac delta function as infinity at zero? select count(distinct service_type) as distinct_service_type from service_table; When we use COUNT and DISTINCT together, Hive always ignores the setting such as mapred.reduce.tasks = 20 for the number of reducers used and uses only one reducer. The GROUP BY clause is used to group all the records in a result set using a particular collection column. STORED AS RCFILE Fixing Hive stats. p_date string Hive multiple distinct on query running slow? user string my favorite data loss bug: mistyping a partition name deletes all your data; complicated queries in a union all will produce erroneous results; hive often generates painfully bad hadoop. ,count(distinct a.user) as AllDist Bug 1329170 - Hive translator - select distinc count(*) is not supported. OK Hive Aggregate Functions Syntax & Description COUNT() Returns the count of all rows in a table including rows containing NULL values When you specify a column as an input, it ignores NULL values in the column for the count. select color, count(*) from deck_of_cards group by color; Hive basically takes the above query to convert it to the map-reduce program by generating corresponding java code and jar file and then executes. The output of count (*) and count (1) is same but the difference is in the time taken to execute the query. tommy 123 2013-12-02 I have a table in Hive and I want to count unique records and all records. INSERT OVERWRITE TABLE dest_Table_B PARTITION(dt='2013-12-02') Set this property: hive.optimize.distinct.rewrite=true Also there is mapper aggregation (mapper can pre-aggregate data also and produce distinct values in the scope of their portion of data - splits) Set this property to allow map-side aggregation: hive.map.aggr=true is not working properly when hive.optimize.countdistinct is true. The difference is in the plan. Hive number of reducers in group by and count(distinct), https://stackoverflow.com/a/51492032/2700344, issues.apache.org/jira/projects/HIVE/issues, Level Up: Creative coding with p5.js – part 1, Stack Overflow for Teams is now free forever for up to 50 users. For Hive 2.2.0 and later, set hive.groupby.position.alias to true (the default is false). – Charles Feb 10 '15 at 9:33 @Charles ANALYZE is a good trick but I have the impression it doesn't work on EXTERNAL tables. Should I say "sent by post" or "sent by a post"? Step1. Thanks for contributing an answer to Stack Overflow! Before HIVE-14251 in release 2.2.0, Hive tries to perform implicit conversion across Hive type groups. When I run. Second query is using intermediate aggrgation and final reducer receives partially aggregated data (distinct values aggregated on previous step). Probabilistic counting fast results, but approximate practical example of using HLL in Hive more theory in the appendix 10. Build new segment. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. group by a.dt. ... and the other five have 5 distinct statuses. There are a lot of similar bug tickets. The row does not mean entire row in the table but it means “row” as per column listed in the SELECT statement. This chapter explains the details of GROUP BY clause in a SELECT statement. By default, Impala only allows a single COUNT(DISTINCT columns) expression in each query.. [jira] [Created] (KYLIN-4805) Hive Global Dictionary - COUNT_DISTINCT(HLL) with multiple parameters Classic List: Threaded ♦ ♦ 1 message ShengJun Zheng (Jira) Reply | Threaded. Current implementation has the limitation that no ORDER BY or window specification can be supported in the partitioning clause for performance reason. XML Word Printable JSON. I want to calculate maximum temperature from temperature_data table corresponding to those years which have at least 2 entries in the table- I tried with below query but it is not working In Apache Hive Tutorial, for grouping particular column values mentioned with the group by Query. OK Complex Counts in Hive This came up on the Hive mailing list and I'm putting it here as a reminder to try it out. August, 2017 adarsh Leave a comment. Connect and share knowledge within a single location that is structured and easy to search. We can have a different type of Clauses associated with Hive to perform different type data manipulations and querying. This came up on the Hive mailing list and I'm putting it here as a reminder to try it out. SELECT [ALL | DISTINCT] select_expr, select_expr, … FROM table_reference [WHERE where_condition] [GROUP BY col_list] [HAVING having_condition] [ORDER BY col_list]] [LIMIT number]; Let’s read about Apache Hive Built-In Functions in detail. STORED AS RCFILE Here's how to do complex count statements to simplify queries. For example, Apache Beam has 5, and Apache Hive has 6. Create cube which contains COUNT_DISTINCT(bitmap) measure. The following table represents records with a measure of various items per day: OK Maybe some reducers are failing. The DISTINCT expression may be used in Aggregate or Remove Duplicates operators. PARTITIONED BY (dt string) For example, Apache Beam has 5, and Apache Hive has … Gives: Blog About Me John Meagher's Ramblings. ii. tommy 111 2013-12-02. Note, Hive supports SELECT DISTINCT * starting in release 1.1.0 . There are a lot of similar bug tickets. eg count distinct will often attempt to push all data through a single reducer; Conditions on … ! Time taken: 0.116 seconds We then also added a row count validation step to Cloverleaf that uses Hive’s table statistics. Seems to only come up when an INSERT is used (Store in Hive operator). I have truncated a particular partition say for example I have truncated Japan partition, now the count of rows is reduced to 700. Step 3. The result seems expected. Let me write a query to group the data by its colour and get its count. Hi, I have a query select x from Y join (Z) as z_1 1. If you do not need precise accuracy, you can produce an estimate of the distinct values for a column by specifying NDV(column); a query can contain multiple instances of NDV(column).To make Impala automatically rewrite COUNT(DISTINCT) expressions to NDV(), enable the APPX_COUNT_DISTINCT … However, I realize that both queries use 370 mappers and 1 reducers and thay have almost the same cumulative CPU seconds. only one distinct user. To remove duplicate values, you can use insert overwrite table in Hive using the DISTINCT keyword while selecting from the original table. Seems to only come up when an INSERT is used (Store in Hive operator). OK Create all arrays of non-negative integers of length N with sum of parts equal to T, If a response to a question was "我很喜欢看法国电影," would the question be "你很喜欢不很喜欢看法国电影？" or "你喜欢不喜欢看法国电影?". LOCATION '/hive//dest_Table_A'; CREATE EXTERNAL TABLE dest_Table_B I am very new to HIVE and have an issue with distinct count and GROUP BY. Partition … If the SELECT has 3 columns listed then SELECT DISTINCT will fetch unique row for those 3 column values only. The query executes using multiple Mappers and one Reduce stage. Tags: hive Description. Vertex failure? Multiple reducers on the same vertex are running independently, isolated and each will produce it's own output, this is why the last stage has single reducer. There are a lot of similar bug tickets. 3. Usage. Is there anything like Schengen area anywhere else in the world? Hive provides SQL type querying language for the ETL purpose on top of Hadoop file system.. Hive Query language (HiveQL) provides SQL type environment in Hive to work with tables, databases, queries. By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. This is valid, but it triggers the PARQUET-1217 bug in the predicate push-down code of the Parquet Java library (parquet-mr). Then If I run the query select distinct country from mytable it displays India,China and Japan even i have no … Column Type Conversion. Although, let’s suppose an employee table. How do I create the left to right CRT refresh effect with material nodes? Join Stack Overflow to learn, share knowledge, and build your career. set mapreduce.input.fileinputformat.split.minsize=67108864; The query executes using multiple Mappers and one Reduce stage. Why am I getting negative allocated mappers in Tez job? hive> select count(distinct a.user) from table_a a; There were row count validation failures even in those tables where the overall row counts were fine. This … Try to run on Tez. Impala may return empty results when DISTINCT or COUNT(DISTINCT ..) expressions are used 1. Supervisor who accepted me for a research internship could not recognize me. Does blocking keywords prevent code injection inside this interactive Python file? I took the Contoso Retail DW sample database from Microsoft and I expanded it quite a bit to get us a more meaningful volume of data. Therefore, the “reduce” method becomes an expensive and all in-memory operation. After trying many datasets observed that Hive is doing Type111Users + Typoe123Users = DistinctUsers which is wrong. STORED AS RCFILE The row does not mean entire row in the table but it means “row” as per column listed in the SELECT statement. ROW FORMAT DELIMITED How to stream data from embedded system to PC fast? However in Hive 0.11.0 and later, columns can be specified by position when configured as follows:. 2013-12-02 2 1 1 2013-12-02 INSERT OVERWRITE TABLE dest_Table_A PARTITION(dt='2013-12-02') count ( 1 ) is faster/optimized than count ( * ) because: count ( * ) has to iterate through all the columns , Step 1) Create a table Table_A If there is a bug, then the version of hive you are using would be important to know since bugs are addressed in each release. Type: Bug Status: Open. FIELDS TERMINATED BY '|' Here's how to do complex count statements to simplify queries. Many queries can be executed on static and dynamic table. Probabilistic counting fast results, but approximate practical example of using HLL in Hive more theory in the appendix 10. Making statements based on opinion; back them up with references or personal experience. select DISTINCT in HIVE. HIVE----count(distinct) over() cannot use the solution, Programmer Sought, the best programmer technical posts sharing site. In groupByExpression columns are specified by name, not by position number. Hivemall implements HyperLogLog++in approx_count_distinct. The editor cannot find a referee to my paper after one year. In the two plans below, the aggregations part in the Output of Group By Operator of Map 1 are different. Total MapReduce CPU Time Spent: 4 seconds 350 msec Total count of records in hive table is 1000. Hive offers several built-in aggregate functions, such as MAX, MIN, AVG, and so on. What would happen if 250 nuclear weapons were detonated within Owens Valley in California? However, one can still count distinct items in a window by using another method. Also it will not fetch DISTINCT value for 1 of the column. ( I am confused why the all use one 1 reducers and I even tried mapreduce.job.reduces but it does not work. Applying DISTINCT and DISTINCTCOUNT does not remove duplicates from Int64 column in Power BI Desktop. How to make electronic systems which work below −40°C (−40°F)? Then If I run the query select distinct country from mytable it displays India,China and Japan even i have no data in the partition country=Japan. Photo Competition 2021-03-29: Transportation. For Hive 2.2.0 and later, set hive.groupby.position.alias to true (the default is false). Actually, query A takes about 1000-1500 seconds while query B takes 500-900 seconds. Are "μπ" and "ντ" indicators that the word didn't exist in Koine/Ancient Greek? ) 1, HIVE-3728 Distinct support in Hive 2.1.0 and later (see HIVE-9534) Distinct is supported for aggregation functions including SUM, COUNT and AVG, which aggregate over the distinct values within each partition. it was posted in 2015. select sex,name -- ,count(DISTINCT id) ,count(DISTINCT id) over (PARTITION BY sex,name) from ani GROUP BY id,sex,name Stricter Types Numeric literals that are not integers, default to DECIMAL in Hive 2. ROW FORMAT DELIMITED And this means they do not have geneiune difference and the time difference may caused by cluster load. Conclusion: Hive gives a count of 2 for distinct users although there is only one distinct user. hive> select * from dest_table_a; Impala may return empty results when DISTINCT or COUNT(DISTINCT ..) expressions are used . Introduction to Hive Group By. One reducer processing too much data may cause a data skew. I prepared a test dataset which will be used on both platforms. Btw, if they all use 1 reducers why do people suggest not to use count(distinct ) and it seems data skew is not avoidable? As of Hive 1.2.0 there is optimization for count(distinct) and you do not need to rewrite query. Hive; HIVE-7400; count and count distinct not correct. That is, the OVER() clause is not allowed at all with this function.. ) I was told that count(distinct ) may result in data skew because only one reducer is used. I'd say hive has a bug or two.. – StephenBoesch Jun 11 '14 at 7:34. , Type123Users int DISTINCT specifies removal of duplicate rows from the result set. In Hive, COUNT (distinct) is a single reducer problem and goes through a massive reduce side sort. The DISTINCT keyword returns unique records from the table. By default, it's true for all 3.x versions. hive> select * from dest_table_b; Here is the schema of the data as it would be inside a SQL Server table: The dataset was extracted into CSV files using UTF-8 encoding. Impala may return empty results when DISTINCT or COUNT(DISTINCT ..) expressions are used. ,count(distinct case when a.type = 111 then a.user else null end) as Type111User Here's how to do complex count statements to simplify queries. count(distinct value)can often cause memory exhausted errors where input data and the cardinality of value are large. Total count of records in hive table is 1000. @user2894829 Check the reducers execution log. Both queries are using the same number of mappers which is expected and single final reducer, which is also expected because you need single scalar count result. make optimizing multi-group by configurable, make optimizing multi-group by configurable. ,count(distinct case when a.type = 111 then a.user else null end) as Type111User Redshift has a count() window function, but it doesn’t support counting distinct items. Test it. Need 2 rows to reproduce this Bug. It contain one normal column, dict_key, that is all distinct literal value for each kylin.dictionary.mr-hive.columns (duplicated literal value are only remain once). Seems to only come up when an INSERT is used (Store in Hive operator). hive is buggy. Step 2. What should I do? ) Basically, we use Hive Group by Query with Multiple columns on Hive tables. ... A bug in Hive’s “Union Distinct” clause (highly unlikely) group by a.dt USE CATALOG hive; SHOW TABLES; DESCRIBE prod_nation; SELECT * FROM prod_nation; Running Queries on Static and Dynamic Tables.
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