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Displaying posts with tag: aggregation (reset)

Fun with Bugs #28 - regression bugs in MySQL 5.6
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2013 was a great year for MySQL Community. New MySQL 5.6 GA release with its increased throughput, scalability and new features as well as more interaction and cooperation with MySQL Community from Oracle side brought us a lot of new perspectives and good feelings over the year.

Unfortunately new MySQL 5.6 GA release also reminded about old and well known problem with new MySQL versions. They all introduce new regression bugs. MySQL 5.6 had not become an exception.

Note that according to good old tradition (that I hope will be followed in 2014) bugs that demonstrate a regression (make some feature that previously worked stop functioning as intended in a new release) are marked with "regression" tag at http://bugs.mysql.com.



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Advantages of weighted lists in RDBMS processing
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A list is simply a list of things. The list has no structure, except in some cases, the length of the list may be known. The list may contain duplicate items. In the following example the number 1 is included twice.

Example list:
1
2
3
1

A set is similar to a list, but has the following differences:
  • The size of the set is always known

  • A set may not contain duplicates

  • You can convert a list to a set by creating a 'weighted list'. The weighted list includes a count column so that you can determine when an item in the list appears more than once:
    1,2
    2,1
    3,1
    
    Notice that there are two number 1 values in the weighted list. In order to make insertions into such a list scalable, consider using partitioning to avoid large indexes.








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    I wrote a new tool that runs aggregation queries over MySQL sharded databases using Gearman.
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    I created a new tool this week:
    http://code.google.com/p/shard-query

    As the name Shard-Query suggests, the goal of the tool is to run a query over multiple shards, and to return the combined results together as a unified query. It uses Gearman to ask each server for a set of rows and then runs the query over the combined set. This isn't a new idea, however, Shard-Query is different than other Gearman examples I've seen, because it supports aggregation.

    It does this by doing some basic query rewriting based on the input query.

    Take this query for example:
    select c2, 
           sum(s0.c1), 
           max(c1) 
     from t1 as s0 
     join t1 using (c1,c2) 
     where c2 = 98818 
     group by c2;
    


    The tool will split this up into two queries.

    This first query will be sent to each shard. Notice











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    Showing entries 1 to 3

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