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Message Posted: Wed, 20 Aug 2002 @ 17:33:45 GMT


     
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Subj:   PI and statistics
 
From:   Hiroshi Takenaka

Hi all,

I have a question on PI and statistics.

From a small table 'orig_tbl' (ca. 700,000rows)
with 3 columns col1, col2 and col3,
I created two new tables with different PI as following:

  create table tblA as (select * from orig_tbl) with data
    primary index (col1);
  collect statistics on tblA column col3;

  create table tblB as (select * from orig_tbl) with data
    primary index (col2);
  collect statistics on tblB column col3;

Then I did:

  explain select * from tblA where col3 = valueX;

My Teradata V2R4.1 system said:

  [snip]
  3) We do an all-AMPs RETRIEVE step from tblA by way
     of an all-rows scan with a condition of (tblA.col3 = valueX)
     into Spool 1, which is built locally on the AMPs.
     The size of Spool 1 is estimated with high confidence to be 1 row.
  [snip]

But it claimed that the size of Spool 1 is estimated to be 4,855 rows, when I did the same on tblB.

The difference may come from the fact that col2 has 700,000 different values (almost unique) while col1 has only 150 different values. But I can't imagine what is actually happening.

Could someone please explain why?


Regards.

Hiroshi Takenaka



     
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