Explain plan question

I was running the following statement through explain plan:

update <table>
set field2 = 'jumbo'
where field1 = decode('&&var1',NULL,field1,'&&var1');

here field1 is the primary key and var1 is variable defined in SQLPLUS.

The explain plane said it would be full table scan. I was curious if that is really true, especially if var1 is defined ? When does the decode() evaluation happen ? If I change the above to:

update <table>
set field2 = 'jumbo'
where field1 = decode('&&var1',NULL,'bozo','&&var1');

Then explain plan says unique index will be used

thanks for any comments
d Received on Thu Feb 09 1995 - 04:41:40 CET