Question

Here in the query when i am trying to execute the OR condition before AND conditions it gives wrong result..as web_filename_username_4hr_ts_201109 also gets included..
Wrong Result Query:

SELECT tablename FROM pg_tables
 WHERE schemaname = 'public'
   and (tablename like 'web%_4hr_ts_%'
       and tablename not like 'webs%_4hr_ts_%'
       and tablename not like 'webr%_4hr_ts_%'
        or tablename like '%web%summary%_4hr_ts_%'
       and tablename not like 'web_filename_username%_4hr_ts_%'
       )
     ;

But when i executed all AND before ORing it gives Correct result…
Correct Result Query:

SELECT tablename
  FROM pg_tables
 WHERE schemaname = 'public'
   and (tablename like 'web%_4hr_ts_%'
    and tablename not like 'webs%_12hr_ts_%'
    and tablename not like 'webr%_4hr_ts_%'
    and tablename not like 'web_filename_username%_4hr_ts_%'
     or tablename like '%web%summary%_4hr_ts_%'
       )

I dont know why….?????????

Answer 1

Because the way you have written it it checks in to blocks.

tablename like 'web%_4hr_ts_%'
and tablename not like 'webs%_4hr_ts_%'
and tablename not like 'webr%_4hr_ts_%'

or

tablename like '%web%summary%_4hr_ts_%'
and tablename not like 'web_filename_username%_4hr_ts_%'

now obviously tablename can not match both '%web%summary%_4hr_ts_%' and 'web_filename_username%_4hr_ts_%' so thats always going to be false.

Here is what explain says about it

 Nested Loop  (cost=0.00..28.55 rows=1 width=64)
   Join Filter: (c.relnamespace = n.oid)
   ->  Seq Scan on pg_class c  (cost=0.00..27.45 rows=1 width=72)
         Filter: ((relkind = 'r'::"char") AND (((relname ~~ 'web%_4hr_ts_%'::text) AND (relname !~~ 'webs%_4hr_ts_%'::text) AND (relname !~~ 'webr%_4hr_ts_%'::text)) OR ((relname ~~ '%web%summary%_4hr_ts_%'::text) AND (relname !~~ 'web_filename_username%_4hr_ts_%'::text))))
   ->  Seq Scan on pg_namespace n  (cost=0.00..1.09 rows=1 width=4)
         Filter: (n.nspname = 'public'::name)

As you can see its added brackets showing that the OR is at the top level. This is why when Im using any OR with AND I always use brackets to ensure the correct evaluation order.

If your ever wondering why a statement doesn’t give you what you expect just stick explain in front of it and see what its going to do.