Question

I noticed when looking at dba_tab_col_statistics for my table (m_CURRENT) that the num_buckets value for column TNE is 75 on one database and 254 on another. DB is Oracle 10g.

This seems to be the main difference between the two tables. Is there a way to get both databases to match num_bucket values?

I have a delete statement that is fast on one database and very slow on another. I know there are several reasons a query plan can differ on two databases. After a lot of analysis, I suspect that getting the slow query database to have the same num_bucket setting could ensure my delete statement does a range_scan (fast) and not a fast_full_scan (slow in this case) on index TNE_idx.

Answer 1

how do you gather stats on both databases, do you have a regular stats gather script? as you can as a one off do this to gather histograms on that one column alone:

begin
dbms_stats.gather_table_stats(user,'M_CURRENT', 
          method_opt=>'for columns TNE size 254', cascade=>false, 
           granularity=>'ALL', degree=>8);
end;
/

the size parameter will set the buckets (if there’s less than that number of distinct values, the result will be less).

The above doesn’t specify estimate_pct so will sample a smaller population of values rather than 100%. if you want 100 percent then specify this in the estimate_pct parameter*), but if you have a regular script, this may get overwritten later on.

* you can check the current sample size by comparing sample_size on the dba_tab_col_statistics to the num_rows on dba_tables