Usually the Cost-Based Optimizer arrives at a reasonable execution plan if it gets the estimates regarding cardinality and data scattering / clustering right (if you want to learn more about that why not watch my Webinar available at "AllThingsOracle.com"?).
Here is an example I've recently come across where this wasn't case - the optimizer obviously preferred plans with a significantly higher cost.
The setup to reproduce the issue is simple:
As in – how come a unique (or primary key) index is predicted to return more than one row using a unique scan, for example (running on 10.2.0.3 – but the same type of thing happens on newer versions):
I’m in the process of writing a number of new presentations and in one I’ve included a favorite little graph of mine that I’ve used over the years to help illustrate the relationship between the cost of using an index vs. the cost of using a Full Table Scan (FTS). It’s occurred to me that I’ve never actually [...]
Basically OP has a query with disjuncted (OR-ed) predicate which started to fail after 126.96.36.199 upgrade with ORA-01790: expression must have same datatype as corresponding expression. Here is a test case (I’ve renamed column and table names cause I’ve used to such naming):
You might have expected the following query ought to run reasonably efficiently, after all it seems to be targeted very accurately at precisely the few rows of information I’m interested in:
select column_name, avg_col_len from dba_subpart_col_statistics where owner = 'TEST_USER' and table_name = 'TEST_COMP' and subpartition_name = 'P_MAX_D'
I’m after some subpartition column stats (so that I can work out whether a subpartition of a local index on a composite partition is roughly the right size) and I’m querying the view by the only columns that seem to be there to allow me to access the data efficiently. Unfortunately the execution plan isn’t doing what I need it to do. The following plan is coming from a small 188.8.131.52 database with up to date statistics:
I’ve previously discussed Virtual Indexes and how they can be used to do basic “what if” analysis if such an index really existed. However, a recent comment on the OTN forums regarding using them to compare index costs made me think a follow-up post regarding the dangers of Virtual Indexes might be warranted. The big advantage of [...]
The basic formula for calculating the costs of a Nested Loop Join is pretty straightforward and has been described and published several times.
In principle it is the cost of acquiring the driving row source plus the cost of acquiring the inner row source of the Nested Loop as many times as the driving row source dictates via the cardinality of the driving row source.
Cost (outer rowsource) + Cost (inner rowsource) * Card (outer rowsource)
Obviously there are cases where Oracle has introduced refinements to the above formula where this is no longer true. Here is one of these cases that is probably not uncommon.
In 1990, when Ken Jacobs hosted the RDBMS campground talks at the Anaheim International Oracle User Week appreciation event, one of the topic areas was whether we (some users representing the Very Large DataBases VLDB of the Oracle world which meant anything north of about 7 GB back then) thought that the rule based optimizer (RBO) was good enough, or whether we needed a cost based optimizer (CBO) for the real applications we were running at enterprise scale to work well. “Oracle’s optimizer is like Mary Poppins. It’s practically perfect in every way. But we do have some cases where it would be helpful for the optimizer to consider the relative sizes of tables and whether a table was local or remote when the plan for joining and filtering is constructed.
I’m very keen on the 11g extended stats feature, but I’ve just discovered a critical weakness in one of the implementation details that could lead to some surprising instability in execution plans. It’s a combination of “column group” statistics and “out of range” predicates. Let’s start with some sample data. (Note: I was running this test on 184.108.40.206):
create table t1 as with generator as ( select --+ materialize rownum id from dual connect by level <= 10000 ) select mod(rownum,100) col1, mod(rownum,10) col2 from generator v1, generator v2 where rownum <= 50000 ; begin dbms_stats.gather_table_stats( ownname => user, tabname =>'T1', method_opt => 'for all columns size 1' ); end; /
Oracle 11g added Extended Statistics support for column groups in order to detect correlated columns for filter predicates using an equal comparison.
Note that Oracle 11g also added the ability to use the number of distinct keys of a composite index as an upper limit for the cardinality estimates for matching column predicates, which means that the optimizer is now capable of detecting correlated columns without the explicit addition of Extended Statistics / Column Groups.