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Thread Adding one field caused 95% drop in blb file size
Tue, Mar 17 2009 8:35 AMPermanent Link

Tony Pomfrett
Hi Tim,

I had a table with 177 records and an associated blob file of 1.1GB.

I altered the table with DBSys (4.25 B1) to add one Date field at the end of the structure which increased the number of fields from 139 to 140 but
decreased the .blb file from 1.1GB down to 58MB.

I'm concerned  that a 95% reduction in file size might involve some data loss. Is there an easy way to find out?

I've checked that no blob fields which previously had data have been set to Null but do I need to check that each non-null blob field matches its
pre-alteration counterpart. Some are images and some are memos. Is there a query that can do this?

Tony.
Tue, Mar 17 2009 8:41 AMPermanent Link

Tony Pomfrett
I just optimised the pre-alteration table without adding the extra field and the 1.1GB .blb file again reduced to 58MB.

Is this a normal reduction and what happened to the missing space?
Tue, Mar 17 2009 9:07 AMPermanent Link

Roy Lambert

NLH Associates

Team Elevate Team Elevate

Tony

Its quite normal. As you add and delete data from a table the space used will not be immediately reclaimed. DBISAM will attempt to reuse the space but this can be difficult with respect to blobs - standard table fields are all the same - blobs vary all over the place. Optimising essentially creates a new version of the table and moves the data over to it leaving all the wasted space behind. Hence the shrinkage from 1.1GB to 58MB.

You must has had a lot of activity in the blob fields probably with some very large blobs being replaced by smaller ones.


Roy Lambert [Team Elevate]
Tue, Mar 17 2009 9:35 AMPermanent Link

Tony Pomfrett
Thanks Roy.

It's hard to imagine that there would have been that much activity with the blob fields. The memo fields wouldn't have changed much and the
other blobs are images (client photos). Maybe somebody added a few 200MB jpegs and later deleted them? Anyway, good to know this is normal.

Tony.
Tue, Mar 17 2009 10:02 AMPermanent Link

Roy Lambert

NLH Associates

Team Elevate Team Elevate

Tony


Or it could be that they added them as bitmaps realised how humongus they were and replaced them with jpegs.

Roy Lambert
Image