==============================================================================
       Topic: ISAM
       Title: Empty Index Block Calculation Lies
   Posted by: Jeff Kreider
    Mail box: JABA/AM
 Date posted: 05/22/93
 Time posted: 8:39 PM
  Times read: 21 
 ------------------------------- Message Text ---------------------------------
 

 Jeff  Kreider,  Consultant is chagrinned. Not only did he write a
 bug,  he  lied  to  you. In "Inside ISAM", appendix D, I describe
 how  to  write  a basic program to anticipate the number of empty
 index  blocks  required  of  ISMBLD  based  on  the    parameters
 prompted.  In  the  analysis,  I  show  that  it  is necessary to
 calculate  the  number  of  blocks required for each of the three
 levels  of  an  ISAM  index.  Each  of  these  formulas  requires
 isolating a number "X" in the form:
 
     qty = X*(seed - some'stuff) + some'stuff - some'other
 
 In  the  process  of  solving  for  X, I point out that it is the
 integer  amount  of  X that is needed and any fractional part may
 be  ignored.  "WRONG,  calculus  breath!"  I  say  to myself. The
 fractional  part  IS  significant.    Ignoring  it could make the
 calculated blocks short by as much as 3 blocks.
 
 Consequently,  I  have  placed  in the UPLOAD: account ISMBLD.BAS
 which  is  a  program built with the correct algorithm (I suspect
 it will be moved, eventually, to the Basic Utilities account).
 
 Those  of  you  who  have  "Inside  ISAM",  the following are the
 details on how to change your copy for accuracy:
 
 Page 151:
 
 WAS:
 
     N=INT((REC'CNT-BLK+1)/HLF'BLK)
 
 SHOULD BE:
 
     N=(REC'CNT-BLK+1)/HLF'BLK
     IF N # INT(N) THEN N=INT(N+1)
 
 WAS:
 
     Z=INT((THIRD'LEVEL - BLK)/HLF'BLK)
 
 SHOULD BE:
 
     Z=(THIRD'LEVEL - BLK)/HLF'BLK
     IF Z # INT(Z) THEN Z=INT(Z+1)
 
 Page 152 at the top:
 
 WAS:
 
     I = INT((SEC'LEVEL - BLK)/HLF'BLK)
 
 SHOULD BE:
 
     I = (SEC'LEVEL - BLK)/HLF'BLK
     IF I # INT(I) THEN I=INT(I+1)
 
 Page 154, Note #2 should read:
 
     Our  assumption  is  that  the  last block contains BLK-1
     entries,  which  are  subtracted  from  the  record count
     (REC'CNT)  to calculate N. Any fractional part of N, will
     cause  another block to be required. This fractional part
     suggests  that  less  than HLF'BLK entries will reside in
     it.  Actually,  this block will also have HLF'BLK entries
     the  entries  in  the last block will be reduced from the
     BLK-1 entries of our assumption.
 
 
 Similar  changes  should be noted in AMSD Journal Article, "ISAM:
 Calculating  the Number of Empty Index Blocks", file 3.3.7 in the
 Software Volume from November, 1985.
 
 I  was  curious why it took so long (8 years!) for the problem to
 materialize.   A case in point is with a 12-byte key because a 12
 byte  key  with  15000  records  will exactly fill the ISAM index
 during  ISMBLD.  Most  people  build a file from 10 to 50% bigger
 than  their  current  needs and rebuild when the file runs out of
 room  or  they  notice  it  is  within  80 to 95% of capacity. In
 either  case,  how  big it should have been is either academic or
 goes unnoticed.
 
 I apologize for any inconvenience this may have caused.
 
 Jeff Kreider,
 Consultant

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