Early instant DDL attempt

[hasan-dot]

Related issue: https://github.com/github/issues-platform/issues/1128

Description

This PR moves the --attempt-instant-ddl check to run before ghost table and binlog streaming setup.

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

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Attempt instant DDL early, before ghost table setup

Problem

When --attempt-instant-ddl is enabled, gh-ost currently performs all the heavy setup work before even trying instant DDL:

1. Start binlog streaming
2. Create changelog table
3. Create ghost table + ALTER it
4. Then attempt ALGORITHM=INSTANT on the original table
5. If it succeeds → clean up the ghost/changelog tables that were just created

For large tables this wastes significant time and resources on setup that gets thrown away when instant DDL succeeds. On extremely large tables (billions of rows), the unnecessary ghost table creation and binlog streaming initialization adds meaningful overhead.

Solution

Move the instant DDL attempt to right after initiateInspector() (which discovers the master connection config) but before initiateStreaming() and initiateApplier(). If instant DDL succeeds, the migration completes immediately without ever creating ghost tables, changelog tables, or starting binlog streaming.

Benchmark results

Benchmarked on M1 Max with MySQL 8.0.42 in Docker (3 iterations each):

Scenario	Instant DDL (early)	Full Row Copy	Speedup
Empty table	25ms	1,123ms	45x
10k rows	27ms	2,659ms	99x
100k rows	38ms	3,622ms	95x

Instant DDL is ~25-38ms regardless of table size — it's a metadata-only operation. The speedup grows with data size, and on production tables with millions of rows this would be the difference between milliseconds and hours.

Benchmark gist: https://gist.github.com/hasan-dot/23697ec67401348f7c4d64f60212374b

CLI flag

Flag	Description
--attempt-instant-ddl	(existing, behavior changed) Try instant DDL early before ghost table setup, fall back to row copy

Migration flow comparison

Before (current):

graph LR
    A[Inspector] --> B[Binlog Streaming]
    B --> C[Ghost Table + Changelog]
    C --> D{Attempt instant DDL}
    D -->|success| E[Done]
    D -->|fail| F[Row Copy]

Loading

After (this PR):

graph LR
    A[Inspector] --> B{Attempt instant DDL}
    B -->|success| C[Done]
    B -->|fail| D[Binlog Streaming]
    D --> E[Ghost Table + Changelog]
    E --> F[Row Copy]

Loading

Instant DDL operations (MySQL 8.0+)

These operations complete in milliseconds regardless of table size (source):

• Adding a column (at end: 8.0.12+; any position: 8.0.29+)
• Dropping a column
• Renaming a column
• Adding/dropping virtual generated columns
• Setting or dropping a column default value
• Modifying the definition of an ENUM or SET column
• Changing the index type
• Renaming a table

Testing

• go build ./... passes
• go vet ./... passes
• New integration tests pass (testcontainers + MySQL 8.0)
• Benchmarks confirm expected speedup

[Copilot]

Pull request overview

This PR optimizes the --attempt-instant-ddl flow by attempting ALGORITHM=INSTANT immediately after inspector initialization (master discovery) and before starting binlog streaming or creating ghost/changelog tables, reducing wasted setup work when instant DDL is possible.

Changes:

• Move the instant DDL attempt earlier in Migrator.Migrate() and add a new attemptInstantDDLEarly() helper.
• Remove Applier.AttemptInstantDDL() (instant attempt no longer depends on applier initialization).
• Add an integration test to assert instant DDL completes without creating ghost/changelog tables, and update flag documentation to reflect the new behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
go/logic/migrator.go	Attempts instant DDL before streaming/applier setup; adds attemptInstantDDLEarly() and removes the old late attempt block.
go/logic/applier.go	Removes the previous AttemptInstantDDL() method from the applier.
go/logic/migrator_test.go	Adds an integration test verifying early instant DDL skips ghost/changelog creation.
doc/command-line-flags.md	Documents the updated “attempt instant DDL early” behavior and motivation.

Comments suppressed due to low confidence (1)

go/logic/migrator_test.go:430

• This SHOW TABLES ... LIKE pattern uses '_' which is a single-character wildcard in SQL LIKE, so it doesn’t strictly mean “table name starts with underscore”. To make the assertion precise and avoid accidental matches, escape underscores in the pattern (or add an explicit ESCAPE clause).

	//nolint:execinquery
	err = suite.db.QueryRow("SHOW TABLES IN test LIKE '_testing_ghc'").Scan(&tableName)
	suite.Require().Error(err, "changelog table should not exist after instant DDL")
	suite.Require().Equal(gosql.ErrNoRows, err)

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