Re: A question regarding Index Sizes (Curiosity mainly!!).

On Tue, 30 Mar 1999 stuartk@au1.ibm.com wrote: > For some time now, I have been intrigued about the "index size" which is > reported by the supplied utilities such as dbschema and dbexport. > > For example: > > { TABLE "dba".table1 row size = 81 number of columns = 3 index size = 12 } > ^^^^^^^^^^^^^^^ > create table "informix".notes > ( > integer1 integer, > smallint1 smallint, > char1 char(75) > ); > revoke all on "dba".notes from "public";> > create index "dba".table1_idx on "dba".notes (integer1); > > >From my understanding in the manuals, the to calculate the size of the > index, > it is the (length of the fields in the index) + 4, so theoretically I was > expecting to see 8 as the index size (integer length 4 + 4). > > After some fiddling, trying out different indexes etc. and seeing what > dbschema was reporting in each case, a formula has come to light of how > the utilities calculate their reported amounts. > > This formula is as follows, where k = sum(length of each field in index): > > index size = k + int(k/2) + 6 > > This works with every table in our schema. > > Just out of curiosity, I was wondering if anyone can enlighten me as to why > this formula is used by the utilities, since I cannot relate it back to the > manuals in any way at all ? > > --StuK. > > Stuart, I asked the same question about a month ago and got only 1 response. I'm not sure to the accuracy of the answer so I will just include the message sent to me. >Hi, Joseph: > >I'm not sure if you ever got an answer to your newsgroup question, but the >size >of an index entry is : > >key size + rowid + delete flag + slot entry > >key size -- in your example (integer), this is 4 bytes >rowid is 4 bytes >delete flag is 1 byte >slot entry is 4 bytes > >So, in your case, the math would be: > >4 + 4 + 1 + 4 = 13 > >Perhaps (i.e., I don't know for sure) dbschema does not report the 1 byte >taken up by the delete flag. > >HTH > >--Jeff > >=========================================================== >Jeff Tyzzer >Senior Consultant >Data Warehouse Consulting Practice >Informix Software, Inc. >Phone: (916) 491-1262 >E-mail: jtyzzer@informix.com >=========================================================== This might shed some light on the subject or maybe someone else can help. Because I've never heard of a delete flag in Informix or slot entry. Joe ------------------------------------------------------------------------- Joseph Cullipher <joseph@cannonexpress.com> Standard Disclaimers Apply -------------------------------------------------------------------------