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Re: Moving data from one set of tables to another?

From: "Sean Davis" <sdavis2(at)mail(dot)nih(dot)gov>
To: howard(at)yankeescientific(dot)com
Cc: pgsql-novice(at)postgresql(dot)org
Subject: Re: Moving data from one set of tables to another?
Date: 2008-09-19 14:58:24
Message-ID: 264855a00809190758l6fa77d16v530f20169e4bcda9@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-novice
On Fri, Sep 19, 2008 at 10:48 AM, Howard Eglowstein
<howard(at)yankeescientific(dot)com> wrote:
> Absolutely true, and if the data weren't stored on the same machine which is
> running the client, I would have worked harder to combine statements. In
> this case though, the server and the data are on the same machine and the
> client application doing the SELECT, INSERT and DELETEs is also on the same
> machine.
>
> I'd like to see how to have done this with combined statements if I ever
> have to do it again in a different setup, but it is working well now. It's
> moved about 1/2 million records so far since last night.

So the 150ms was per row?  Not to belabor the point, but I have done
this with tables with tens-of-millions of rows in the space of seconds
to minutes (for the entire move, not per row), depending on the exact
details of the table(s).  No overnight involved.  The network is one
issue (which you have avoided by being local), but the encoding and
decoding overhead to go to a client is another one that is entirely
avoided.  When you have some free time, do benchmark, as I think the
difference could be substantial.

Sean

> Sean Davis wrote:
>>
>> On Fri, Sep 19, 2008 at 7:42 AM, Howard Eglowstein
>> <howard(at)yankeescientific(dot)com> wrote:
>>
>>>
>>> So you'd agree then that I'll need 7 SQL statements but that I could
>>> stack
>>> the INSERT and the first SELECT if I wanted to? Cool. That's what I ended
>>> up
>>> with in C code and it's working pretty well. I did some indexing on the
>>> database and got the whole transaction down to about 150ms for the
>>> sequence.
>>> I guess that's as good as it's going to get.
>>>
>>
>> Keep in mind that the INSERT [...] SELECT [...] is done server-side,
>> so the data never goes over the wire to the client.  This is very
>> different than doing the select, accumulating the data, and then doing
>> the insert and is likely to be much faster, relatively.  150ms is
>> already pretty fast, but the principle of doing as much on the server
>> as possible is an important one when looking for efficiency,
>> especially when data sizes are large.
>>
>> Glad to hear that it is working.
>>
>> Sean
>>
>>
>>>
>>> Sean Davis wrote:
>>>
>>>>
>>>> On Thu, Sep 18, 2008 at 7:28 PM, Howard Eglowstein
>>>> <howard(at)yankeescientific(dot)com> wrote:
>>>>
>>>>
>>>>>
>>>>> What confuses me is that I need to do the one select with all three
>>>>> tables
>>>>> and then do three inserts, no? The results is that the 150 fields I get
>>>>> back
>>>>> from the select have to be split into 3 groups of 50 fields each and
>>>>> then
>>>>> written into three tables.
>>>>>
>>>>>
>>>>
>>>> You do the insert part of the command three times, once for each new
>>>> table, so three separate SQL statements.  The select remains basically
>>>> the same for all three, with only the column selection changing
>>>> (data_a.* when inserting into new_a, data_b.* when inserting into
>>>> new_b, etc.).  Just leave the ids the same as in the first set of
>>>> tables.  There isn't a need to change them in nearly every case.  If
>>>> you need to add a new ID column, you can do that as a serial column in
>>>> the new tables, but I would stick to the original IDs, if possible.
>>>>
>>>> Sean
>>>>
>>>>
>>>>
>>>>>
>>>>> What you're suggesting is that there is some statement which could do
>>>>> the
>>>>> select and the three inserts at once?
>>>>>
>>>>> Howard
>>>>>
>>>>> Sean Davis wrote:
>>>>>
>>>>>
>>>>>>
>>>>>> You might want to look at insert into ... select ...
>>>>>>
>>>>>> You should be able to do this with 1 query per new table (+ the
>>>>>> deletes, obviously).  For a few thousand records, I would expect that
>>>>>> the entire process might take a few seconds.
>>>>>>
>>>>>> Sean
>>>>>>
>>>>>>     On Thu, Sep 18, 2008 at 6:39 PM, Howard Eglowstein
>>>>>> <howard(at)yankeescientific(dot)com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> Somewhat empty, yes. The single set of 'data_' tables contains 3
>>>>>>> years
>>>>>>> worth
>>>>>>> of data. I want to move 2 years worth out into the 'new_' tables.
>>>>>>> When
>>>>>>> I'm
>>>>>>> done, there will still be 1 year's worth of data left in the original
>>>>>>> table.
>>>>>>>
>>>>>>> Howard
>>>>>>>
>>>>>>> Carol Walter wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> What do you want for your end product?  Are the old tables empty
>>>>>>>> after
>>>>>>>> you
>>>>>>>> put the data into the new tables?
>>>>>>>>
>>>>>>>> Carol
>>>>>>>>
>>>>>>>> On Sep 18, 2008, at 3:02 PM, Howard Eglowstein wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> I have three tables called 'data_a', 'data_b' and 'data_c' which
>>>>>>>>> each
>>>>>>>>> have 50 columns. One of the columns in each is 'id' and is used to
>>>>>>>>> keep
>>>>>>>>> track of which data in data_b and data_c corresponds to a row in
>>>>>>>>> data_a.  If
>>>>>>>>> I want to get all of the data in all 150 fields for this month (for
>>>>>>>>> example), I can get it with:
>>>>>>>>>
>>>>>>>>> select * from (data_a, data_b, data_c) where data_a.id=data_b.id
>>>>>>>>> AND
>>>>>>>>> data_a.id = data_c.id AND timestamp >= '2008-09-01 00:00:00' and
>>>>>>>>> timestamp
>>>>>>>>> <= '2008-09-30 23:59:59'
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>
>>>>>>
>>>>>>>>>
>>>>>>>>> What I need to do is execute this search which might return several
>>>>>>>>> thousand rows and write the same structure into 'new_a', 'new_b'
>>>>>>>>> and
>>>>>>>>> 'new_c'. What i'm doing now in a C program is executing the search
>>>>>>>>> above.
>>>>>>>>> Then I execute:
>>>>>>>>>
>>>>>>>>> INSERT INTO data_a (timestamp, field1, field2 ...[imagine 50 of
>>>>>>>>> them])
>>>>>>>>> VALUES ('2008-09-01 00:00:00', 'ABC', 'DEF', ...);
>>>>>>>>> Get the ID that was assigned to this row since 'id' is a serial
>>>>>>>>> field
>>>>>>>>> and
>>>>>>>>> the number is assigned sequentially. Say it comes back as '1'.
>>>>>>>>> INSERT INTO data_b (id, field1, field2 ...[imagine 50 of them])
>>>>>>>>> VALUES
>>>>>>>>> ('1', 'ABC', 'DEF', ...);
>>>>>>>>> INSERT INTO data_c (id, field1, field2 ...[imagine 50 of them])
>>>>>>>>> VALUES
>>>>>>>>> ('1', 'ABC', 'DEF', ...);
>>>>>>>>>
>>>>>>>>> That moves a copy of the three rows of data form the three tables
>>>>>>>>> into
>>>>>>>>> the three separate new tables.
>>>>>>>>> From the original group of tables, the id for these rows was, let's
>>>>>>>>> say,
>>>>>>>>> '1234'. Then I execute:
>>>>>>>>>
>>>>>>>>> DELETE FROM data_a where id='1234';
>>>>>>>>> DELETE FROM data_b where id='1234';
>>>>>>>>> DELETE FROM data_c where id='1234';
>>>>>>>>>
>>>>>>>>> That deletes the old data.
>>>>>>>>>
>>>>>>>>> This works fine and gives me exactly what I wanted, but is there a
>>>>>>>>> better
>>>>>>>>> way? This is 7 SQL calls and it takes about 3 seconds per moved
>>>>>>>>> record
>>>>>>>>> on
>>>>>>>>> our Linux box.
>>>>>>>>>
>>>>>>>>> Any thoughts or suggestions would be appreciated.
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>>
>>>>>>>>> Howard
>>>>>>>>>
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>>>>>>>>>
>>>>>>>>>
>>>>>>>>
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>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
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>
>

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Subject: Re: Moving data from one set of tables to another?
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Subject: Re: Moving data from one set of tables to another?

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