Plan a database engine migration

Engine migrations rarely fail on the bulk copy. They fail on semantics that differ quietly: case-insensitive comparisons that become case-sensitive, zero dates and unsigned integers with no equivalent, sequences not reset after the load, different default isolation levels, query plans that change for the worst queries, and a cutover with no tested way back. A credible plan finds those differences before the copy and makes the cutover boring.

Plan a migration from [SOURCE] to [TARGET]. Only if [DATA_SIZE] is given: Data size and write rate: [DATA_SIZE] Only if [DOWNTIME_BUDGET] is given: Downtime budget: [DOWNTIME_BUDGET]

1. If the data size or the downtime budget is not stated above, or you do not have the schema, ask for them under "Inputs needed" and write the rest of the plan with each dependent choice labelled as an assumption. Ask also for the features in use (stored procedures, triggers, full-text search, JSON, spatial), the application stack and ORM, and the top queries by load.
2. Audit incompatibilities for this pair of engines: data types (booleans, unsigned integers, date and time zones, zero dates, enums, text and binary sizes), character sets and collations including case sensitivity, auto-increment versus identity or sequences, NULL versus empty-string handling, SQL dialect (upsert, limit, group-by strictness, quoting, functions), procedures and triggers, full-text search, JSON operators, default transaction isolation and locking behaviour, and implicit casts.
3. Choose the copy approach from size and downtime: an offline dump and load when the window allows; otherwise a bulk load followed by change data capture to stay in sync until cutover. Name candidate tools and why. Avoid application dual-writes unless you explain how consistency is guaranteed.
4. Phase the work: schema conversion, a test load, application changes behind a switch, performance testing of the top queries on the target, a rehearsal of the full cutover, then production.
5. Write the cutover runbook: stop or freeze writes, drain replication lag to zero, verify, reset sequences, switch connections, smoke test, decision point. Give each step an owner role and duration, and compare the total to the downtime budget.
6. Verification: row counts per table, checksums per chunk on normalised values, sampled row comparison, and application-level comparison of read results.
7. Rollback: how to return to the source after writes have landed on the target (reverse replication or a replay plan), the triggers for rolling back, and the deadline after which you roll forward instead.

• Be specific to [SOURCE] and [TARGET]. Do not list incompatibilities that do not apply to this pair.
• Do not invent table names or sizes. Use the information given and label assumptions.
• A cutover without a rehearsed rollback is a risk to state plainly, not a footnote.
• Separate what you verified from what you inferred. Mark inferences as such.
• When you do not know, say "I don't know" once and state what would settle it.

Summary

Approach, expected downtime, and the top three risks.

Inputs needed

Bullets, or "None".

Incompatibilities

A table: area, behaviour in the source, behaviour in the target, action.

Approach

The copy method and tools, and why.

Phases

A table: phase, work, exit criteria.

Cutover runbook

Numbered steps with owner role and duration, plus the go or no-go checks.

Verification

The checks and their pass criteria.

Rollback

The mechanism, triggers and deadline.

Risks

Bullets with mitigations.