Raghu On Tech

Pro tips on managing data at scale

How To Find Missing Indexes On Foreign Key Columns ?

by | Jul 10, 2018 | 1 comment

Problem Statement:

You are trying to figure out if there are any Foreign key constraints in your database with out the corresponding Index. If you are working in an OLTP environment you already know most of your query patterns include a join on primary key (parent table) and foreign key(s) (child tables). It is important to know and address any of these missing indexes in development/lower environments before they become a problem in production database environments. As basic as it may sound, I have discovered these missing indexes once in a while.

 

Solution:

Solution is an easy one. You run the below SQL and get the list of Foreign Key constraints with out the corresponding indexes. Once you identify these missing indexes you need to use your application/business knowledge to ask yourself if you need to create these indexes. If you are working with a conventional application, answer to this question will most probably be yes. If you are working with an edge case application where parents don’t talk to children then you got a weird family ūüôā

select substr(riinfo.tabschema,1,25) as tabschema,
          substr(riinfo.tabname,1,25) as tabname,
          substr(riinfo.constname,1,25) as constname,
          substr(riinfo.colname,1,25) as colname,
          substr(idxinfo.indname,1,25) as indname
from (select a.tabschema, a.tabname, a.constname, b.colname
          from syscat.tabconst a,
          syscat.keycoluse b
                       where a.type = 'F'
                       and a.tabname = b.tabname
                       and a.tabschema = b.tabschema
                       and a.constname = b.constname) riinfo
left join
          (select a.tabschema,a.tabname,a.indname,b.colname
          from syscat.indexes a,
          syscat.indexcoluse b
                      where a.indschema = b.indschema
                      and a.indname = b.indname) idxinfo
                                   on ( riinfo.tabschema = idxinfo.tabschema
                                   and riinfo.tabname = idxinfo.tabname
                                   and riinfo.colname = idxinfo.colname)
                                   where idxinfo.indname is null order by riinfo.tabname;

 

When I run this SQL against my database I get the below output, which tells me that DB2 explain tables do not have corresponding indexes for their foreign keys. This might not be that big of a problem if your explain tables do not grow.

However if you are keeping track of say for e.g. your top 10 SQL statements performance over a 5 year period of time and you save their explain information into these explain tables, then you may need to create indexes to avoid performance problems for any reporting that you may perform on these tables.

 

select  substr(riinfo.tabschema,1,25) as tabschema, substr(riinfo.tabname,1,25) as tabname, 
substr(riinfo.constname,1,25) as constname, substr(riinfo.colname,1,25) as colname, 
substr(idxinfo.indname,1,25) as indname from (select a.tabschema, a.tabname, a.constname, 
b.colname from syscat.tabconst a, syscat.keycoluse b 
where a.type = 'F' and a.tabname = b.tabname and a.tabschema = b.tabschema and a.constname = b.constname) riinfo 
left join (select a.tabschema,a.tabname,a.indname,b.colname from syscat.indexes a, syscat.indexcoluse b 
where a.indschema = b.indschema and a.indname = b.indname) idxinfo 
on ( riinfo.tabschema = idxinfo.tabschema and riinfo.tabname = idxinfo.tabname and riinfo.colname = idxinfo.colname) 
where idxinfo.indname is null order by riinfo.tabname

TABSCHEMA                 TABNAME                                  CONSTNAME                                          COLNAME                   INDNAME
------------------------- ---------------------------------------- -------------------------------------------------- ------------------------- -------------------------
TRAVELER                  ADVISE_INDEX                             SQL131029095034720                                 RUN_ID                    -
TRAVELER                  ADVISE_MQT                               SQL131029095142470                                 RUN_ID                    -
TRAVELER                  ADVISE_PARTITION                         SQL131029095217570                                 RUN_ID                    -
TRAVELER                  ADVISE_TABLE                             SQL131029095253200                                 RUN_ID                    -
TRAVELER                  EXPLAIN_ACTUALS                          SQL131029095430650                                 EXPLAIN_LEVEL             -
TRAVELER                  EXPLAIN_ACTUALS                          SQL131029095430650                                 EXPLAIN_REQUESTER         -
TRAVELER                  EXPLAIN_ACTUALS                          SQL131029095430650                                 EXPLAIN_TIME              -
TRAVELER                  EXPLAIN_ACTUALS                          SQL131029095430650                                 OPERATOR_ID               -
TRAVELER                  EXPLAIN_ACTUALS                          SQL131029095430650                                 SECTNO                    -
TRAVELER                  EXPLAIN_ACTUALS                          SQL131029095430650                                 SOURCE_NAME               -
TRAVELER                  EXPLAIN_ACTUALS                          SQL131029095430650                                 SOURCE_SCHEMA             -
TRAVELER                  EXPLAIN_ACTUALS                          SQL131029095430650                                 SOURCE_VERSION            -
TRAVELER                  EXPLAIN_ACTUALS                          SQL131029095430650                                 STMTNO                    -
TRAVELER                  EXPLAIN_ARGUMENT                         SQL131029094618810                                 EXPLAIN_REQUESTER         -
TRAVELER                  EXPLAIN_ARGUMENT                         SQL131029094618810                                 SOURCE_NAME               -
TRAVELER                  EXPLAIN_ARGUMENT                         SQL131029094618810                                 SOURCE_SCHEMA             -
TRAVELER                  EXPLAIN_ARGUMENT                         SQL131029094618810                                 SOURCE_VERSION            -
TRAVELER                  EXPLAIN_DIAGNOSTIC_DATA                  SQL131029094918880                                 EXPLAIN_REQUESTER         -
TRAVELER                  EXPLAIN_DIAGNOSTIC_DATA                  SQL131029094918880                                 SOURCE_NAME               -
TRAVELER                  EXPLAIN_DIAGNOSTIC_DATA                  SQL131029094918880                                 SOURCE_SCHEMA             -
TRAVELER                  EXPLAIN_DIAGNOSTIC_DATA                  SQL131029094918880                                 SOURCE_VERSION            -
TRAVELER                  EXPLAIN_OBJECT                           SQL131029094632060                                 EXPLAIN_REQUESTER         -
TRAVELER                  EXPLAIN_OBJECT                           SQL131029094632060                                 SOURCE_NAME               -
TRAVELER                  EXPLAIN_OBJECT                           SQL131029094632060                                 SOURCE_SCHEMA             -
TRAVELER                  EXPLAIN_OBJECT                           SQL131029094632060                                 SOURCE_VERSION            -
TRAVELER                  EXPLAIN_PREDICATE                        SQL131029094735980                                 EXPLAIN_REQUESTER         -
TRAVELER                  EXPLAIN_PREDICATE                        SQL131029094735980                                 SOURCE_NAME               -
TRAVELER                  EXPLAIN_PREDICATE                        SQL131029094735980                                 SOURCE_SCHEMA             -
TRAVELER                  EXPLAIN_PREDICATE                        SQL131029094735980                                 SOURCE_VERSION            -
TRAVELER                  EXPLAIN_STREAM                           SQL131029094809960                                 EXPLAIN_REQUESTER         -
TRAVELER                  EXPLAIN_STREAM                           SQL131029094809960                                 SOURCE_NAME               -
TRAVELER                  EXPLAIN_STREAM                           SQL131029094809960                                 SOURCE_SCHEMA             -
TRAVELER                  EXPLAIN_STREAM                           SQL131029094809960                                 SOURCE_VERSION            -

  33 record(s) selected.

 

Hope you enjoyed this short blog post. If you have any similar SQL tricks that you use on a regular basis, share them in the comments section below.

 

PS: You may find this stackoverflow discussion useful for understanding different types of joins.

 

1 Comment

  1. Raghu

    One of the readers brought it to my attention that this query is showing duplicate column names for the same table. I figured out that the root cause of this problem is a missing join between syscat.tabconst and syscat.keycoluse.

    and a.tabschema = b.tabschema

    I added the above join and data is looking much better now.

    Reply

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