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3.9.4 Partitioning and Constraint Exclusion

Constraint exclusion is a query optimization technique that improves performance for partitioned tables defined in the fashion described above. As an example:

SET constraint_exclusion = on;
SELECT count(*) FROM measurement WHERE logdate >= DATE 
  '2008-01-01';

Without constraint exclusion, the above query would scan each of the partitions of the measurement table. With constraint exclusion enabled, the planner will examine the constraints of each partition and try to prove that the partition need not be scanned because it could not contain any rows meeting the query's WHERE clause. When the planner can prove this, it excludes the partition from the query plan.

You can use the EXPLAIN command to show the difference between a plan with constraint_exclusion on and a plan with it off. A typical unoptimized plan for this type of table setup is:

SET constraint_exclusion = off;
EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= 
  DATE '2008-01-01';
                    QUERY PLAN
--------------------------------------------------
 Aggregate  (cost=158.66..158.68 rows=1 width=0)
   ->  Append  (cost=0.00..151.88 rows=2715 width=0)
         ->  Seq Scan on measurement  
         (cost=0.00..30.38 rows=543 width=0)
               Filter: (logdate >= '2008-01-01'::date)
         ->  Seq Scan on measurement_y2006m02 measurement  
         (cost=0.00..30.38 rows=543 width=0)
               Filter: (logdate >= '2008-01-01'::date)
         ->  Seq Scan on measurement_y2006m03 measurement  
         (cost=0.00..30.38 rows=543 width=0)
               Filter: (logdate >= '2008-01-01'::date)
...
         ->  Seq Scan on measurement_y2007m12 measurement  
         (cost=0.00..30.38 rows=543 width=0)
               Filter: (logdate >= '2008-01-01'::date)
         ->  Seq Scan on measurement_y2008m01 measurement  
         (cost=0.00..30.38 rows=543 width=0)
               Filter: (logdate >= '2008-01-01'::date)

Some or all of the partitions might use index scans instead of full-table sequential scans, but the point here is that there is no need to scan the older partitions at all to answer this query. When we enable constraint exclusion, we get a significantly cheaper plan that will deliver the same answer:

SET constraint_exclusion = on;
EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= 
  DATE '2008-01-01';
                    QUERY PLAN
--------------------------------------------------
 Aggregate  (cost=63.47..63.48 rows=1 width=0)
   ->  Append  (cost=0.00..60.75 rows=1086 width=0)
         ->  Seq Scan on measurement  
         (cost=0.00..30.38 rows=543 width=0)
               Filter: (logdate >= '2008-01-01'::date)
         ->  Seq Scan on measurement_y2008m01 measurement  
         (cost=0.00..30.38 rows=543 width=0)
               Filter: (logdate >= '2008-01-01'::date)

Note that constraint exclusion is driven only by CHECK constraints, not by the presence of indexes. Therefore it isn't necessary to define indexes on the key columns. Whether an index needs to be created for a given partition depends on whether you expect that queries that scan the partition will generally scan a large part of the partition or just a small part. An index will be helpful in the latter case but not the former.

The default (and recommended) setting of constraint_exclusion is actually neither on nor off, but an intermediate setting called partition, which causes the technique to be applied only to queries that are likely to be working on partitioned tables. The on setting causes the planner to examine CHECK constraints in all queries, even simple ones that are unlikely to benefit.