FILES

* execution.dir

** cachetable.scm

External queries  are materialised on remote  pool in a
table used  as a result  cache.  These tables  are then
accessed  by FDW.   This is  done to  avoid performance
issue with  FDW and  to create a  distributed execution
plan.

On pools:
 CREATE TABLE <LOCAL CACHE TABLE> 
 AS <QUERY> ;

On the master:
 CREATE FOREIGN TABLE <FOREIGN CACHE TABLE> ( 
   <TYPE> 
   ... 
 ) SERVER <POOL> 
   OPTION ( TABLE_NAME <LOCAL CACHE TABLE> ) ;

** describe.scm

Foreign tables  creation need knowledge  of table types
(columns names of a  given query). A function is needed
to obtain the columns name and their types from a given
SQL query.

Beware  of anonymous  or duplicate  columns in  the SQL
query as this prevents the remote table creation.

** execute.scm

An execution plan is  a structure describing actions to
be  done.  Actions  are  SQL queries  with  their  pool
location.

These actions may be either:
- sequential:  (sequential <plan> ...)
- parallel:    (parallel <plan> ...)

SQL execution is done asynchronously.

** externalquery.scm

Definition: A  subquery is an external one  if and only
if it could be executed as it is on the remote pool.

A subquery is an external query if we have:
- all its tables on the same pool
- there is no references to free variables

For instance, this subquery is not external because of O1:
SELECT * FROM Object O1, Object O2
WHERE (SELECT count(*) FROM Source S 
       WHERE O1.objectid = S.objectid) > 100 ;

This one is made of 2 external queries: 
SELECT * FROM Source S1
WHERE objectid = 1234 
UNION ALL
SELECT * FROM Source S2
WHERE objectid = 1234 ;

We search  in the  rewrited query for  maximal external
queries. Then  we replace these queries  with a foreign
table and we create this foreign table on the pool .

** framedef.scm

Frames are  used to determine subqueries  type. A frame
describes the set of reachable variables from a lexical
point.

** frame.scm

This file defines operations on frames:
- Search a table name in a frame
- Search a column name in a frame
- Obtain all columns and their types

** freevar.scm

Search all free variables in a subquery.

** from.scm

Creates a frame from a FROM clause.

** parallel.scm

Executes a list of plans in parallel (with one thread per plan for the moment).

** placeholder.scm

A  placeholder  defines a  hole  in  a  subquery to  be
replaced later.  Placeholders have an  identity to find
them.

** plan.scm

A  plan is  a recursive  data structure  describing all
action steps to be executed.

plan ::= (sequential-plan plan ...) |
         (parallel-plan plan ...)   |
         (action sql pool)

The  scheduler  takes a  query,  extracts all  external
subqueries and creates an execution plan.