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From navision help:
Date Compress Item Ledger Batch Job
This batch job compresses item ledger entries, that is, combines them so that they take up less space in the database. You can compress entries from closed fiscal years only, and only item ledger entries whose Open field contains "No."
The compression works by combining several old entries into one new entry. For example, item ledger entries from previous fiscal years can be compressed so that there is only one credit and one debit entry per account per month. The amount in the new entry is the sum of all the compressed entries. The date assigned is the starting date for the period that is compressed, such as the first day of the month (if the entries are compressed by month). After the compression you can still see the net change for each account in the previous fiscal year.
After the compression the contents of the following fields are always retained: Posting Date, Item No., Entry Type, Quantity, Invoiced Quantity, Remaining Quantity, Sales Amount (Actual), Cost Amount (Actual), Cost Amount (Expected). With the Retain Field Contents facility, you can also retain the contents of these additional fields: Document No., Location Code, Serial No., Source Type, Source No., Source Posting Group, Sales/Purch. Person, Transaction Type, Transport Method, Country Code, Global Dimension 1 Code and Global Dimension 2 Code.
The number of entries that result from a Date Compress batch job depends on how many filters you set, which fields are combined, and which period length you choose. There will always be at least one entry. When the batch job is finished, you can see the result in the Date Compr. Register, which is found under General Ledger, Periodic Activities, Date Compression.
Warning
Date Compression deletes entries, so you should always make a backup copy of the database before you run the batch job.
You can define what will be included in the batch job by setting filters. You can select additional fields on the Item Ledger Entry tab by pressing F3 and then clicking the AssistButton that appears to the right of the Field field. In addition, the Options tab contains other fields in which you must specify conditions for running the batch job. Fill in the fields as follows:
Item Ledger Entry
Item No.: Select the numbers of the items to be included in the batch job. To see the existing item numbers, click the Filter field and then click the AssistButton that appears to the right.
Inventory Posting Group: Select the inventory posting groups to be included in the batch job. To see the existing inventory posting groups, click the Filter field and then click the AssistButton that appears to the right.
Click the up-arrow at the lower left-hand corner of the window to see the result of the filters you have defined for the table.
Options
Starting Date: Enter the first date to be included in the date compression. The compression will affect all item ledger entries from this date to the Ending Date.
Ending Date: Enter the last date to be included in the date compression. The compression will affect all item ledger entries from the Starting Date to this date.
Period Length: Select the length of the period whose entries will be combined. Click the AssistButton to the right of the field to see the options. If you selected the period length Quarter, Month or Week, only entries with a common accounting period are compressed.
Posting Description: Here you can enter a text that will accompany the entries that result from the compression. The default description is "Date Compressed."
Retain Field Contents: Place checks in the boxes if you want to retain the contents of certain fields even though the entries are compressed. The more fields you select, the more detailed the compressed entries will be. If you do not select any of these fields, the batch job will create one entry per day, week, or another period, according to the period selected in the Period Length field.
Click OK to start the batch job. If you do not want to run the batch job now, click Cancel to close the window.
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From navision help: