PostgreSQL snapshot support

Use this matrix before selecting postgresSnapshotAdapter; it records the PostgreSQL facts Qubu v1 can encode and the cases that need a later server version policy.

Import the adapter from the PostgreSQL snapshot subpath:

import { createSchemaSnapshot } from "qubu/snapshot"
import {
  createSchemaSnapshot as createPostgresSnapshot,
  postgresSnapshotAdapter,
} from "qubu/snapshot/postgres"

const snapshot = createPostgresSnapshot(appSchema)
// Equivalent: createSchemaSnapshot(appSchema, { adapter: postgresSnapshotAdapter })

The schema dialect extends Qubu's existing postgresql query dialect, so both snapshot metadata and unsafeSchemaSql() use postgresql consistently.

Support matrix

Schema fact	PostgreSQL v1 behavior
Portable storage	Emits PostgreSQL declarations: INTEGER, NUMERIC, TEXT, BOOLEAN, DATE, TIMESTAMP, UUID, JSONB, BIGINT, and BYTEA.
Native storage	Preserves a non-empty declaration tagged postgresql exactly. Other dialect tags fail.
Literals	Encodes finite numbers, strings, booleans, bigint, and NULL without query parameters.
Defaults	Canonical literals, branded deterministic expressions, and explicit external behavior are retained. Column references in defaults fail.
Identity	always and by-default identity metadata stays separate from generated expressions.
Generated columns	Stored expressions are supported. Virtual generated columns fail with a capability diagnostic.
Keys and checks	Primary keys, strict unique keys, ordinary unique constraints, foreign keys, and checks retain names, timing, actions, and expressions. Checks cannot be deferrable.
Foreign-key match	simple and full are retained. partial fails because PostgreSQL does not implement it.
Nullable uniqueness	nulls: 'distinct' is portable. nulls: 'not-distinct' needs a PostgreSQL 15-or-newer policy and is rejected by v1.
Indexes	Ordered terms, expressions, predicates, included columns, uniqueness, and candidate-key evidence are retained. PostgreSQL method, concurrency, operator class, and storage-parameter extensions are encoded under dialect.
Names	Table, column, constraint, and index names are checked against PostgreSQL's 63-byte identifier limit. Relation names are checked for collisions across tables and indexes.

The adapter does not connect to PostgreSQL or emit DDL. It produces deterministic data for the qubu/snapshot decoder. The package-wide ownership map shows the separate schema and application boundaries.

Diagnostics

Use the non-throwing form when a schema may contain a server-specific feature:

import { tryCreateSchemaSnapshot } from "qubu/snapshot/postgres"

const result = tryCreateSchemaSnapshot(appSchema)
if (!result.ok) {
  for (const issue of result.diagnostics) {
    console.error(issue.path.join("."), issue.code, issue.message)
  }
}

Capability checks run before common traversal. The common serializer still owns ordering, cross-reference checks, canonical encoding, and strict snapshot validation.