# Plan: Bidirectional Transformation Support

## Goal

Make the transformation pipeline direction-aware. Currently hardcoded to MSSQL → PG; add support for PG → MSSQL by applying inverse transformations when `SourceDbType == "postgres"`.

Excluded: `to_storage` Azure blob upload (not reversible).

---

## Hardcoded wiring to fix

| File | Line | Change |
|---|---|---|
| `cmd/go_migrate/process.go` | 51 | Branch on `SourceDbType`: `"sqlserver"` → `NewMssqlTransformer`, `"postgres"` → `NewPostgresTransformer` |
| `cmd/go_migrate/main.go` | 166–167 | Branch on source/target type for both `TableAnalyzer` selections |

---

## Transformations

### Forward (MSSQL → PG) — unchanged

| Column type | Function | File |
|---|---|---|
| `uniqueidentifier` | `mssqlUuidToBigEndian` | `utils.go:9` |
| `geometry`/`geography` | `wkbToEwkbWithSrid` | `utils.go:25` |
| `datetime`/`datetime2` | `ensureUTC` | `utils.go:57` |

### Inverse (PG → MSSQL) — new

| PG system type | Action |
|---|---|
| `uuid` | `bigEndianToMssqlUuid`: re-swap bytes [0-3], [4-5], [6-7] |
| `geometry` | `ewkbToMssqlGeo(v, false)`: strip SRID → WKB → `WkbToUdtGeo` |
| `geography` | `ewkbToMssqlGeo(v, true)`: strip SRID → WKB → `WkbToUdtGeo` |
| `timestamp`/`timestamptz` | no-op |

**Geometry note**: MSSQL rejects plain WKB via bulk protocol. Must use `mssqlclrgeo.WkbToUdtGeo(wkb, isGeography)` (already in go.mod). PG extractor already emits EWKB via `ST_AsEWKB()`.

---

## New utility functions (`transformers/utils.go`)

### `bigEndianToMssqlUuid(v []byte) []byte`
```
out[0..3]  = v[3,2,1,0]
out[4..5]  = v[5,4]
out[6..7]  = v[7,6]
out[8..15] = v[8..15]
```

### `ewkbToMssqlGeo(ewkb []byte, isGeography bool) ([]byte, error)`
1. Read byte-order flag from `ewkb[0]`
2. Read geometry type word bytes [1..4]
3. If SRID flag (`0x20000000`) is set: strip bytes [5..8], clear flag in type word
4. Call `mssqlclrgeo.WkbToUdtGeo(wkb, isGeography)`

---

## New files

### `transformers/postgres.go`
```go
func NewPostgresTransformer(...) *Transformer {
    // same signature as NewMssqlTransformer
    // calls computePostgresTransformationPlan instead
    // does NOT call computeStorageTransformationPlan
}
```

### `computePostgresTransformationPlan` in `transformers/plan.go`
Iterates `sourceColTypes` (from PG analyzer), applies inverse closures by system type.

---

## PostgreSQL table analyzer stubs to implement (`table_analyzers/postgres.go`)

Required for PG-as-source partitioned extraction:

### `EstimateTotalRows`
```sql
SELECT reltuples::bigint FROM pg_class
  JOIN pg_namespace ON pg_namespace.oid = pg_class.relnamespace
  WHERE pg_namespace.nspname = $schema AND pg_class.relname = $table
```
Fallback to `COUNT(*)` if `reltuples < 0`.

### `QueryMaxMinFromColumn`
```sql
SELECT MIN("col"), MAX("col") FROM "schema"."table"
```

### `CalculatePartitionRanges`
Use min/max from above + `rowsPerPartition` to compute boundaries. Mirror the logic from `MssqlTableAnalyzer.CalculatePartitionRanges`.

---

## Test cases

### TC-1: `bigEndianToMssqlUuid` — round-trip
- Input: run `mssqlUuidToBigEndian` on a known 16-byte MSSQL UUID → produces PG UUID
- Assert: `bigEndianToMssqlUuid(pgUUID)` == original MSSQL UUID bytes
- Also assert nil input → nil output (no panic)

### TC-2: `bigEndianToMssqlUuid` — known vector
- Input: `[0x6b,0xa7,0xb8,0x10, 0x9d,0xad, 0x11,0xd1, 0x80,0xb4,0x00,0xc0,0x4f,0xd4,0x30,0xc8]` (RFC 4122 nil UUID variant)
- Assert: bytes [0-3] are reversed, [4-5] reversed, [6-7] reversed, [8-15] identical

### TC-3: `ewkbToMssqlGeo` — geometry round-trip
- Input: generate a polygon via `go-geom` + `wkb.Marshal` → plain WKB
- Forward: run `wkbToEwkbWithSrid` → EWKB
- Inverse: run `ewkbToMssqlGeo(ewkb, false)` → CLR/UDT bytes
- Assert: no error, output is non-empty `[]byte`

### TC-4: `ewkbToMssqlGeo` — nil input
- Input: nil
- Assert: returns nil, nil (no panic)

### TC-5: `ewkbToMssqlGeo` — EWKB without SRID flag
- Input: plain WKB (no SRID flag set)
- Assert: function still calls `WkbToUdtGeo` and returns without error

### TC-6: Transformer factory selection
- Given `SourceDbType == "postgres"` → `NewPostgresTransformer` is selected
- Given `SourceDbType == "sqlserver"` → `NewMssqlTransformer` is selected

---

## Files changed (summary)

1. `cmd/go_migrate/process.go` — transformer factory branch
2. `cmd/go_migrate/main.go` — analyzer selection branch
3. `internal/app/etl/transformers/utils.go` — 2 new functions
4. `internal/app/etl/transformers/plan.go` — `computePostgresTransformationPlan`
5. `internal/app/etl/transformers/postgres.go` *(new)*
6. `internal/app/etl/table_analyzers/postgres.go` — 3 stub implementations