Re: Exhaused sequence

During off hours create a table that has all the missing numbers from
your column.  Select your new customer number by selecting
min(missing_number) from that table and then have an after insert
trigger that deletes the min(missing_number).  When your new table has
0 rows you're done.

It'll get rid of the slow inserts from real-time searching for
available numbers but your customer should re-think their business
rule.  Unless they plan on limiting their growth they need to expand
the column.

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