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I know this issue can be solved using unique key constraint on column and some insert command variation.

But I exactly want to know that at last why the following approach goes wrong sometimes and a duplicate entry is added:

  1. a value is checked in the column
  2. if record returns is > 0 then update is issued, otherwise simple insert command

Most of the times it works. You can say more than 98%, but why does it fail sometimes?

I just want to understand where the glitch is in this approach because record existence is checked first.

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it's hard to say without some sort of DDL. What is the purpose of the value >1? At first glance, it should be >=1, but again, don't know for sure without know what the purpose of your column is, and how the value changes. –  Derek Downey Aug 30 '11 at 17:25
    
ahh, looks like the >1 was a typo. –  Derek Downey Aug 30 '11 at 17:49
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migrated from serverfault.com Aug 30 '11 at 17:16

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1 Answer

up vote 3 down vote accepted

One scenario in which this would fail depends on how busy (or unlucky) you are in timing.

  • Session A does the select on the value, sees that it should INSERT
  • Session B does a select on the value before session A does the required insert or update of the value constraint
  • Session A inserts row, updates value
  • Session B inserts row, updates value

It's all a matter of timing (as suggested by your 98% works). You could acquire a WRITE LOCK for Session A on the table you are inserting AND on the table you are updating the value, which will block Session B until Session A releases the lock.

Alternatively, you could go ahead and let the DB handle the locking automatically by creating a UNIQUE INDEX and issuing a INSERT .. ON DUPLICATE UPDATE (recommended)

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I understand about that unlucky timing but when already there are two records.. one time bad timing happened already. Then why/how third time same entry could be added? And yes I know its solution as I mentioned in my question but i wanted to know the glitch in this technique. –  Rick James Aug 30 '11 at 18:02
    
Can't really say without some DDL and the exact queries you're calling, but the logic looks fine. Tempted to call application foul. (AKA you have a bad filter when selecting the value that fails sometimes). Another good reason to let the database handle it. –  Derek Downey Aug 30 '11 at 18:06
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