1. How the ENVS and QRES stacks work (example)
3rd Country Conference, February 2007
How the ENVS and QRES stacks work (example) by
Prof. Kazimierz Subieta
SBA/SBQL pages:

2. Query addressing a tiny database
Query: For each department get its reference and the average sal of its employees.
SBQL: Dept join avg(employs.Emp.sal)
Tiny database:
Expected result: bag{ struct{i17, 2500}, struct{i22, 1450} }

3. Execution scenario Dept join avg( ( employs . Emp ) . sal ) join
1st Dept
1st employs
1st Emp
2nd employs
2nd Emp
2nd Dept
3rd employs
3rd Emp
join
1st dot
2nd dot
avg

4. Sample query Dept join avg( ( employs . Emp ) . sal ) Start empty ENVS
Emp(i1) Emp(i5) Emp(i9)
Dept(i17) Dept(i22)
empty
ENVS
QRES

5. In closing …
Please note that this evaluation scenario concerns the formal semantics only, executed by an abstract machine.
Our goal is clear, universal and free of ambiguities semantic specification rather than performance.
In real implementation there are many obvious optimizations.
C.f. our implementations in various projects.
The evaluation process is also the subject of many sophisticated optimizations (rewriting, indices, caching,…) .
The entire process is executed in main memory, hence the computations are not critical.
Critical computations concern the access to external storage and network, and avoiding unnecessary (redundant) computations.
This is just the subject of major SBA optimization methods.