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Message Posted: Fri, 21 Nov 2003 @ 12:56:26 GMT


     
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Subj:   A couple of questions on an explain text
 
From:   Narayan Murthy

I have a couple of questions w.r.t the explain statement below :

1. In Step 2 of the explain we see 3 writes and a read, but in Step 3 we see 4 writes and a read - why is this difference.

2. There is another DBC table(DBC.Next) that gets updated apart from TVM, TVFields, Dbase, Indexes & Accessrights - but why isn't there a mention of DBC.Next in the Explain output


Narayan

-------------
explain
create table t1(
c1 int,
c2 char);
Explanation
--------------------------------------------------
 
  1)First, we lock CHK.t1 for exclusive use.  
  2)Next, we lock a distinct DBC."pseudo table" for write on a RowHash for deadlock prevention, we lock a distinct DBC."pseudo table" for read on a RowHash for deadlock prevention, we lock a distinct DBC."pseudo table" for write on a RowHash for deadlock prevention, we lock a distinct DBC."pseudo table" for write on a RowHash for deadlock prevention.  
  3)We lock DBC.DBase for read on a RowHash, we lock DBC.TVM for write on a RowHash, we lock DBC.TVFields for write on a RowHash, we lock DBC.AccessRights for write on a RowHash, we lock DBC.Indexes for write on a RowHash.  
  4)We execute the following steps in parallel.  
   
  1) We do a single-AMP ABORT test from DBC.DBase by way of the unique primary index.
 
   
  2) We do a single-AMP ABORT test from DBC.TVM by way of the unique primary index.
 
   
  3) We do an INSERT into DBC.TVFields (no lock required).
 
   
  4) We do an INSERT into DBC.TVFields (no lock required).
 
   
  5) We do an INSERT into DBC.Indexes (no lock required).
 
   
  6) We do an INSERT into DBC.TVM (no lock required).
 
   
  7) We INSERT default rights to DBC.AccessRights for CHK.t1.
 
  5)We create the table header.  
  6)Finally, we send out an END TRANSACTION step to all AMPs involved in processing the request.  
  ->No rows are returned to the user as the result of statement 1.  



     
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