Understanding SQLTXPLAIN Health-Checks
SQLTXPLAIN (SQLT) collects diagnostics details about a SQL statement that either performs poorly or it produces unexpected results (also known as wrong results). In addition to that, it also performs over a hundred health-checks around the SQL statement being analyzed. The results of these so called health-checks are displayed on the main HTML report under the “Observations” section. Within each observation there is description of the meaning and in some cases a pointer to known bugs or notes. Since there are so many health-checks, it happens that every once in a while we want to dig further into WHY we get this “observation”. If that is your case and you understand SQL and PL/SQL, I invite you to “hack” into the SQLT source code and see how this health-check is triggered.
Finding WHY SQLT triggered a particular Health-Check
Assume you get this “observation” in your main HTML report “Table contains 2 column(s) referenced in predicates where the number of distinct values does not match the number of buckets.“.
Open sqlt/install/sqcpkgh.pkb (this h is for health-checks) and search for a portion of this text “number of distinct values does not match the number of buckets”. You will find a piece of code that looks like this:
-- 10174050 frequency histograms with less buckets than ndv BEGIN SELECT COUNT(*) INTO l_count FROM sqlt$_dba_all_table_cols_v WHERE statement_id = p_tab_rec.statement_id AND owner = p_tab_rec.owner AND table_name = p_tab_rec.table_name AND histogram = 'FREQUENCY' AND num_distinct <> num_buckets AND in_predicates = 'TRUE'; IF l_count > 0 THEN s_obs_rec.type_id := E_TABLE; s_obs_rec.object_type := 'TABLE'; s_obs_rec.object_name := p_tab_rec.owner||'.'||p_tab_rec.table_name; s_obs_rec.observation := 'Table contains '||l_count||' column(s) referenced in predicates where the number of distinct values does not match the number of buckets.'; s_obs_rec.more := 'Review <a href="#tab_cols_cbo_'||p_tab_rec.object_id||'">column statistics</a> for this table and look for "Num Distinct" and "Num Buckets". If there are values missing from the frequency histogram you may have Bug 10174050. If you are referencing in your predicates one of the missing values the CBO can over estimate table cardinality, and this may produce a sub-optimal plan. As a workaround: alter system/session "_fix_control"=''5483301:off'';'; ins_obs; END IF; END;
In this case the health-check derives from view sqlt$_dba_all_table_cols_v. You can find all view definitions inside file sqlt/install/sqcvw.sql. This file shows that view sqlt$_dba_all_table_cols_v selects from tables sqlt$_dba_tab_cols and sqlt$_dba_nested_table_cols.
There is a predicate on the health-check that reads “num_distinct <> num_buckets“. So this condition is what triggered this particular health-check.
In some cases, the column driving the health-check is an addition to the base DBA or GV$ views, so in such cases you may have to search for that column in one of these two packages: sqlt/install/sqcpkgd.pkb or sqlt/install/sqcpkgt.pkb. Where the “d” stands for Diagnostics data collection and the “t” for Transformation of diagnostics data.
When you get a SQLT health-check that is of your concern and you need to find out WHY it was raised, you may need to look into SQLT source code. Always start on the “h” module and continue either in view definitions or in the two packages that seed special columns. Those two packages “d” and “t” are the ones responsible for data collection and transformations.
SQLT source code is transparent. It is written in an easy to follow style. Feel free to reverse engineer any part of it. No mysteries, no hidden pieces, no black magic…