Stage 7: Additional DDL

Stage Objective

Implement schema modification capabilities through ALTER TABLE and DROP TABLE.

Conceptual Algorithms

• DROP TABLE: Locate the table in the catalog schema, delete the B-Tree root page, mark all associated pages as free in the free-list (so they can be reused), and remove the catalog entry.
• ALTER TABLE: Implement adding new columns. For schema versioning, existing rows won't be physically rewritten immediately. Instead, the schema catalog records the default value for the new column, and the read path will pad rows that are missing the column.

Implementation Checklist

• Add DROP, ALTER, ADD, COLUMN tokens to Lexer.
• Parse DROP TABLE and ALTER TABLE ADD COLUMN commands.
• Update the catalog storage to remove table references for DROP.
• Implement page deallocation (adding pages to the free-list).
• Update the schema catalog to support schema versioning / adding columns.
• Ensure SELECT reads old tuples properly with default padding for new columns.

Expected Contract

When DROP TABLE users; is executed, subsequent queries on users should fail with "Table not found", and disk space should be marked as reusable. When ALTER TABLE users ADD COLUMN age INT; is executed, existing rows will return NULL (or a default) for age.