Identifying Current Record

Question

Identifying Current Record

rka

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Points: 1599
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March 6, 2014 at 4:45 pm

#283076

I have a snapshot table of about 15 million records in the form of:
InvoiceIDLineItemIDSnapshotDateAmount
1 1 20140101 12
1 2 20140102 14
1 3 20140103 17
2 1 20140101 10
2 2 20140102 5
1 2 20140105 15
1 3 20140105 20
I want to create an additional column called Current as shown below:
InvoiceIDLineItemIDSnapshotDateAmount Current
1 1 20140101 12 1
1 2 20140102 14 0
1 3 20140103 17 0
2 1 20140101 10 1
2 2 20140102 5 1
1 2 20140105 15 1
1 3 20140105 20 1
How can we write a query to achieve this while keeping in mind:
- We do not want to do unnecessary record lookups and Updates
- We only update records that corresponds to new entries. For example, we should not touch the record for InvoiceID = 2 in the above example
Thanks

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Lowell SSC Guru Points: 323482 More actions · Answer 1

don't try to do it in the table. create a view or query that uses row_number() OVER (Partition by...order by..)

if you try and do it in the table itself, you will run into a huge performance hit in your trigger, which has to rearrange data with any insert update or delete.

so just don't do that.

use that as a sub query, and whatever has the lowest/highest number gets the 'Current'd description as a case statement.

wild guess:

SELECT CASE WHEN RW = 1 THEN 'Current' ELSE '' END AS isCurrentIndicator,

*

FROM (

SELECT

ROW_NUMBER() OVER (PARTITION BY InvoiceID ORDER BY InvoiceID,LineItemID) AS RW,

InvoiceID,

LineItemID,

SnapshotDate,

Amount

FROM YourTable) SubQueryAlias

ORDER BY InvoiceID,LineItemID

Lowell

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ScottPletcher SSC Guru Points: 100262 More actions · Answer 2

I'd definitely add a bit column to the table. Your data and your statements seem contradictory, but I think when a new row is added that becomes current, you could just update the old row to turn off the current flag. Edit: the appropriate index should make this update relatively painless.

No way I personally would use ROW_NUMBER() or any other such mechanism, since I wouldn't want to risk sorting 15M+ rows, or any significant part thereof, every time I went to read them.

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rka SSCommitted Points: 1599 More actions · Answer 3

I also don't like to use Partitions and Ranks etc due to the sorting overhead

The default value for the new column will be set to 1 thus will allow new records to become active

But what is the best way to detect the old records. I believe I still have to go through all the records via Snapshot_Date. Still can't figure out the best way to extract older records

ScottPletcher SSC Guru Points: 100262 More actions · Answer 4

rka (3/6/2014)
But what is the best way to detect the old records. I believe I still have to go through all the records via Snapshot_Date. Still can't figure out the best way to extract older records

I have no idea. You haven't given any specific rules on how to determine what is current and what no longer is.

SQL DBA,SQL Server MVP(07, 08, 09) "Money can't buy you happiness." Maybe so, but it can make your unhappiness a LOT more comfortable!

Evil Kraig F SSC Guru Points: 100851 More actions · Answer 5

I'd take this as a two phase approach.

First, you need to get your current data into the format you want. No matter what you do, you're going to do a massive table update at least once. I recommend you port it to a new table and swap the table objects if you have the space.

To find the correct current values, you need to use the ROW_NUMBER() technique described by Lowell to activate the most recent invoice/line item combination.

Then, you need to include an INSERT trigger for further updates. Said trigger needs to make sure the same invoice/lineitem is distinct in each inclusion. UPDATE the table to turn off any existing invoiceID/lineitem items with current = 1 (as Scott mentioned, use an index to easily locate these) that exist before allowing the trigger to finish the insert.

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Grant Fritchey SSC Guru Points: 397721 More actions · Answer 6

You might want to take a look at this article I wrote in SimpleTalk. It goes through how to retrieve versioned data[/url] from the table. Assuming you can identify the latest record based on that date column, the techniques outlined here should work for you, and should perform well (we had millions and millions of records and performance was great) as long as you build out your indexes appropriately.

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