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From:Zhangzhigang Date:May 8 2012 2:26am
Subject:回复: 回复: Why is creating indexes f
aster after inserting massive data rows?
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Karen...

The mysql does not use this approach what you said which is complicated.

I  agree with ohan De Meersman.


________________________________
 发件人: Karen Abgarian <abvk@stripped>
收件人: mysql@stripped 
发送日期: 2012年5月8日, 星期二, 上午 1:30
主题: Re: 回复: Why is creating indexes faster after inserting massive data rows?
 
Hi, 

A couple cents to this. 

There isn't really a million of block writes.   The record gets added to the block,
but that gets modified in OS cache if we assume MyISAM tables and in the Innodb buffer if
we assume InnoDB tables.   In both cases, the actual writing does not take place and
does not slow down the process.    What does however happen for each operation,
is processing the statement, locating the entries to update in the index, index block
splits and , for good reason, committing.  

When it comes to creating an index, what needs to happen, is to read the whole table and
to sort all rows by the index key.   The latter process will be the most determining
factor in answering the original question, because for the large tables the sort will have
to do a lot of disk I/O.    The point I am trying to make is there will be
situations when creating indexes and then inserting the rows will be faster than creating
an index afterwards.   If we try to determine such situations, we could notice that
the likelihood of the sort going to disk increases with the amount of distinct values to
be sorted.   For this reason, my choice would be to create things like
primary/unique keys beforehand unless I am certain that everything will fit in the
available memory. 

Peace
Karen



On May 7, 2012, at 8:05 AM, Johan De Meersman wrote:

> ----- Original Message -----
> 
>> From: "Zhangzhigang" <zzgang_2008@stripped>
> 
>> Ok, Creating the index *after* the inserts, the index gets created in
>> a single operation.
>> But the indexes has to be updating row by row after the data rows has
>> all been inserted. Does it work in this way?
> No, when you create an index on an existing table (like after a mass insert), what
happens is that the engine does a single full tablescan and builds the index in a single
pass, which is a lot more performant than updating a single disk block for every record,
for the simple reason that a single disk block can contain dozens of index entries. 
> 
> Imagine that you insert one million rows, and you have 100 index entries in a disk
block (random numbers, to make a point. Real numbers will depend on storage, file system,
index, et cetera). Obviously there's no way to write less than a single block to disk -
that's how it works. 
> 
> You can update your index for each record in turn. That means you will need to do 1
million index - and thus block - writes; plus additional reads for those blocks you don't
have in memory - that's the index cache. 
> 
> Now, if you create a new index on an existing table, you are first of all bypassing
any index read operations - there *is* no index to read, yet. Then the system is going to
do a full tablescan - considered slow, but you need all the data, so there's no better way
anyway. The index will be built - in-memory as much as possible - and the system will
automatically prefer to write only complete blocks - 10.000 of them. That's the exact
same number of index blocks, but you only write each block once, so that's only 10.000
writes instead of 1.000.000. 
> 
> Now there's a lot more at play, things like B-tree balancing and whatnot, but that's
the basic picture. 
> 
> -- 
> 
> Bier met grenadyn 
> Is als mosterd by den wyn 
> Sy die't drinkt, is eene kwezel 
> Hy die't drinkt, is ras een ezel 


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Thread
Why is creating indexes faster after inserting massive data rows?Zhangzhigang7 May
  • Re: Why is creating indexes faster after inserting massive data rows?Ananda Kumar7 May
    • 回复: Why is creating indexes faster after inserting massive data rows?Zhangzhigang7 May
  • Re: Why is creating indexes faster after inserting massive data rows?Johan De Meersman7 May
    • 回复: Why is creating indexes faster after inserting massive data rows?Zhangzhigang7 May
      • Re: 回复: Why is creating indexes faster after inserting massive data rows?Alex Schaft7 May
        • 回复: 回复: Why is creating indexes faster after inserting massive data rows?Zhangzhigang7 May
          • Re: 回复: 回复: Why is creating indexes faster after inserting massive data rows?Claudio Nanni7 May
            • too nice not to share it!hsv2 Oct
          • Re: 回复: 回复: Why is creating indexes faster after inserting massive data rows?Mihamina Rakotomandimby30 May
      • Re: 回复: Why is creating indexes faster after inserting massive data rows?Claudio Nanni7 May
        • 回复: 回复: Why is creating indexes faster after inserting massive data rows?Zhangzhigang7 May
      • Re: 回复: Why is creating indexes faster after inserting massive data rows?Johan De Meersman7 May
        • RE: 回复: Why is creating indexes faster after inserting massive data rows?Rick James7 May
        • Re: 回复: Why is creating indexes faster after inserting massive data rows?Karen Abgarian7 May
          • 回复: 回复: Why is creating indexes faster after inserting massive data rows?Zhangzhigang8 May
            • Re: 回复: 回复: Why is creating indexes faster after inserting massive data rows?Karen Abgarian8 May
              • 回复: 回复: 回复: Why is creating indexes faster after inserting massive data rows?Zhangzhigang8 May
                • Re: 回复: 回复: 回复: Why is creating indexes faster after inserting massive data rows?Johan De Meersman8 May
                  • 回复: 回复: 回复: 回复: Why is creating indexes faster after inserting massive data rows?Zhangzhigang8 May
                  • Re: 回复: 回复: 回复: Why is creating indexes faster after inserting massive data rows?Karen Abgarian8 May
                    • 回复: 回复: 回复: 回复: Why is creating indexes faster after inserting massive data rows?Zhangzhigang9 May
              • RE: 回复: 回复: Why is creating indexes faster after inserting massive data rows?Rick James10 May
            • Re: 回复: 回复: Why is creating indexes faster after inserting massive data rows?Johan De Meersman8 May
        • Re: 回复: Why is creating indexes faster after inserting massive data rows?Karen Abgarian7 May
    • RE: Why is creating indexes faster after inserting massive datarows?Rick James7 May
      • 回复: Why is creating indexes faster after inserting massive data rows?Zhangzhigang9 May
        • RE: 回复: Why is creating indexes faster after inserting massive data rows?Rick James9 May
          • Re: 回复: Why is creating indexes faster after inserting massive data rows?Claudio Nanni9 May
          • 回复: 回复: Why is creating indexes faster after inserting massive data rows?Zhangzhigang10 May