Editing Architecture decision records (section)

== 4. ADR-003 — Divisors Stored in Database ==

'''Date:''' 2025 (inferred)<br />
'''Status:''' Accepted<br />
'''Deciders:''' Original developer<br />
'''Source:''' [INFERRED from algorithm and SQL design]

<span id="context-2"></span>
=== Context ===

The trial-division sieve needs a list of previously found primes to use as divisors. These could be held in a COBOL working-storage table (an array), in a flat file, or in the database.

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=== Decision Drivers ===

* Demonstrate database interaction as part of the algorithm (pedagogical goal).
* Keep the COBOL working-storage footprint small — avoid a large in-memory prime array.
* Persist divisors so the report can run independently of the generation run.

<span id="options-considered-2"></span>
=== Options Considered ===

{|
!width="24%"| Option
!width="39%"| Description
!width="18%"| Pros
!width="18%"| Cons
|-
| A
| In-memory COBOL table
| Fastest; no I/O
| Fixed size; lost between runs; defeats the DB demonstration goal
|-
| B
| Flat file
| Persistent; simple
| Requires file I/O; not the DB demonstration goal
|-
| C
| Database (chosen)
| Persistent; demonstrates DB use; divisors and output in same store
| One SQL query per trial division step — potentially very slow
|}

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=== Decision ===

Option C — each divisor is fetched from the <code>primes</code> table by ident using a point SELECT. Generated primes accumulate in the same table as the divisors.

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=== Consequences ===

'''Positive:''' - Demonstrates the full three-tier pattern end-to-end. - All state is in the database; generation can be interrupted and (in principle) resumed.

'''Negative / trade-offs:''' - Performance is heavily dependent on the <code>SELECT prime WHERE ident = :n</code> query. Without an index on <code>ident</code>, this becomes a sequential scan that gets slower as the table grows. - The design requires prime 2 to be pre-seeded in the table before the algorithm starts, since the first divider fetch assumes ident=2 exists.

'''Risks:''' - No index on <code>ident</code> is defined in <code>primes_schema.sql</code> (see NFR-PERF-03). This is the most significant performance risk in the system.

<span id="compliance-check-2"></span>
=== Compliance check ===

Run <code>\d primes.primes</code> in psql; verify an index on <code>ident</code> is present. If absent, add: <code>CREATE INDEX idx_primes_ident ON primes.primes (ident);</code>


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