perf(tapd): optimize migration script 20230411 for PostgreSQL (22s → 4s)

[narrowizard]

Problem

#8676

Root cause:

• Each column rename operation called HasColumn() twice (checking old and new column names)
• Each HasColumn() call queries information_schema for all column metadata
• Each RenameColumn() triggered a deferred SELECT to clear PostgreSQL's query plan cache
• Total: ~270 information_schema queries + 135 cache clearing SELECTs

Solution

Optimized the migration by:

1. Cache column metadata: Fetch all column names once per table using GetColumns() instead of calling HasColumn() for each column
2. Batch cache clearing: Clear PostgreSQL cached plan once per table instead of after each rename
3. Handle dialect differences:
   - PostgreSQL: Execute separate ALTER TABLE ... RENAME COLUMN statements (required by syntax)
   - MySQL: Use single ALTER TABLE with multiple CHANGE COLUMN clauses

Performance Impact

Metric	Before	After	Improvement
Execution time	22s	4s	5.5x faster
information_schema queries	~270	3	90x reduction
Cache clearing SELECTs	135	3	45x reduction

Testing

Performance comparison

Database	Before (s)	After (s)
PostgreSQL
MySQL

Schema validation

mysql

Postgres