http://people.planetpostgresql.org/dfetter/index.PHP?/archives/80-Approximate-Counts.html
通过这种方法，我们可以快速的估算一个表，视图的记录数，当然也包括带条件的查询中，最终结果的返回集。
例如：
postgres=# EXPLAIN (FORMAT JSON) SELECT 1 FROM  t limit 1;
                QUERY PLAN                 
-------------------------------------------
 [                                        +
   {                                      +
     "Plan": {                            +
       "Node Type": "Limit",+
       "Startup Cost": 0.00,+
       "Total Cost": 0.01,+
       "Plan Rows": 1,+
       "Plan Width": 0,+
       "Plans": [                         +
         {                                +
           "Node Type": "Seq Scan",+
           "Parent Relationship": "Outer",+
           "Relation Name": "t",+
           "Alias": "t",+
           "Startup Cost": 0.00,+
           "Total Cost": 14425.00,+
           "Plan Rows": 1000000,+
           "Plan Width": 0                +
         }                                +
       ]                                  +
     }                                    +
   }                                      +
 ]
(1 row)

postgres=# EXPLAIN (FORMAT JSON) SELECT 1 FROM  t;
           QUERY PLAN           
--------------------------------
 [                             +
   {                           +
     "Plan": {                 +
       "Node Type": "Seq Scan",+
       "Relation Name": "t",+
       "Alias": "t",+
       "Total Cost": 14425.00,+
       "Plan Rows": 1000000,+
       "Plan Width": 0         +
     }                         +
   }                           +
 ]
(1 row)

通过函数，将Plan Rows转换成输出：
postgres=# CREATE OR REPLACE FUNCTION countit(name,name)             
RETURNS float4         
LANGUAGE plpgsql AS        
$$DECLARE             

    v_plan json;              

BEGIN                    

    EXECUTE format('EXPLAIN (FORMAT JSON) SELECT 1 FROM %I.%I',$1,$2)

        INTO v_plan;                                                                     

    RETURN v_plan #>> '{0,Plan,"Plan Rows"}';

END;

$$;
CREATE FUNCTION
使用这种方法就可以快速评估所有表和视图的行数了。
postgres=# SELECT
    relname AS "table",CASE WHEN relkind = 'r'
        THEN reltuples
        ELSE countit(n.nspname,relname)
    END AS "approximate_count"
FROM
    pg_catalog.pg_class c
JOIN
    pg_catalog.pg_namespace n ON (
        c.relkind IN ('r','v') AND
        c.relnamespace = n.oid                                                         
    );

另外，还有一种更加简便的方法是输出pg_class.reltuples，但是这个字段的值analyze（包括auto analyze）后才有的，
而使用explain 的方法，还可以适用没有统计信息的情况。

[参考]
http://people.planetpostgresql.org/dfetter/index.PHP?/archives/80-Approximate-Counts.html