Re: Optimze usage of immutable functions as relation

Hello,

I reworked a patch to make more stable in different cases. I decided to
use simplify_function instead of eval_const_expression to prevent
inlining of the function. The only possible outputs of the
simplify_function are Const node and NULL.

Also, I block pre-evaluation of functions with types other than
TYPTYPE_BASE, cause there is no special logic for compound (and others)
values yet.

There is still a problem with memory leak in case of simplified
arguments. The only way I see is a creation of temporary memory context,
but it cost some performance. Maybe we can store simplified arguments
in the pointed function itself for later use. But eval_const_expression
and friends doesn't change the content of the nodes inside the tree, it
generates new nodes and returns it as a result.

The last point to mention is a fixed plan for the query in the initial
letter of the thread. As I mentioned before, new versions of the patch
replace var not with a function call, but with a function execution
result. After the patch, the following plan is used instead of Nested
Loop with Sequence Scan:

explain select '|'||subject||'|', ts_rank_cd(body_tsvector,q) from messages, to_tsquery('english',
'tuple&header&overhead')q where body_tsvector @@ q limit 10;
 
                                             QUERY PLAN
----------------------------------------------------------------------------------------------------
 Limit  (cost=224.16..266.11 rows=3 width=36)
   ->  Nested Loop  (cost=224.16..266.11 rows=3 width=36)
         ->  Function Scan on q  (cost=0.00..0.01 rows=1 width=0)
         ->  Bitmap Heap Scan on messages  (cost=224.16..266.04 rows=3 width=275)
               Recheck Cond: (body_tsvector @@ '''tupl'' & ''header'' & ''overhead'''::tsquery)
               ->  Bitmap Index Scan on message_body_idx  (cost=0.00..224.16 rows=3 width=0)
                     Index Cond: (body_tsvector @@ '''tupl'' & ''header'' & ''overhead'''::tsquery)

-- 
Aleksandr Parfenov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company