Schema Changes

Table Before Change

Start from a tiny table before any schema step.

highlighted = computed this step

Table before change

Start with a tiny table. A schema change is easier to see when the before table is visible.

start with table\text{start with table}

Show the starting table

The starting table has 2 rows and 2 columns.

rows=2\text{rows}=2

Schema-change examples are tiny finite table transforms; SQL dialects, online migration algorithms, locks, performance, ORMs, compliance, and operational runbooks are out of scope.

Table before change: beforeidname1Ada2Bo ordered schema stepsstepopnonenone step snapshotsstepopcolumnsrows0start[id, name]2 final tableidname1Ada2Bo changed columnskindcolumnsadded[]removed[]kept[id, name] changed rowsrowbeforeafternonenonenone safety factsfactvaluestepCount0rowCount2columnCount2changedRowCount0

No steps yet

There are no schema steps in this first view, so the final table matches the starting table.

before equals after\text{before equals after}

Summary

Every later lesson compares back to this small table.

small pinned table\text{small pinned table}