Add document for planning postgres schema

docs/postgres-planning.md

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+# Planning for implementing a postgres repo for pict-rs
+
+FullRepo currently depends on a number of specific repo traits Many of these are simple key -> value
+mappings that can be done with columns or tables, but some are more complicated (like background job
+processing).
+
+Some of the existing repo methods could be consolidated to better represent the data and operations
+that need to be performed. This should happen _before_ impelementing the postgres repo.
+
+
+### HashRepo
+This is a little complicated because one of the things a HashRepo can do is return a stream of
+hashes from the repo. This can likely be implemented as a batch-retrieval operation that fetches
+1000 hashes at once and then drains them on each call to `poll_next`
+
+This is also probably made up of multiple tables, and can reuse some other tables. It's likely that
+some of this functionality can be consolidated with AliasRepo. In both usages, relate_hash and
+relate_alias are called together.
+
+Create could also maybe be updated to take the identifier as an argument, since it doesn't make
+sense to have a hash without an identifier. This might affect the ingest process' ordering
+
+methods:
+- size
+- hashes
+- create
+- relate_alias
+- remove_alias
+- aliases
+- relate_identifier
+- relate_variant_identifier
+- variant_identifier
+- remove_variant
+- relate_motion_identifier
+- cleanup
+
+```sql
+CREATE TABLE hashes (
+    hash BYTEA PRIMARY KEY,
+    identifer TEXT NOT NULL,
+    motion_identifier TEXT,
+);
+
+CREATE TABLE variants (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    hash BYTEA REFERENCES hashes(hash) ON DELETE CASCADE,
+    variant TEXT NOT NULL,
+    identifier TEXT NOT NULL
+);
+
+CREATE UNIQUE INDEX hash_variant_index ON variants (hash, variant);
+```
+
+
+### AliasRepo
+Used to relate Aliases to Hashes, and to relate Delete Tokens to Aliases. Hashes are always bytes,
+but could be hex-encoded. postgres's `bytea` type can work with hex-encoding on storage and
+retrieval so maybe this can be used. Delete Tokens are not always UUIDs, even though they have been
+UUIDs in all recent versions of pict-rs.
+
+methods:
+- create
+- relate_delete_token
+- relate_hash
+- hash
+- cleanup
+
+This can probably be simplified. If `create` took a `hash` as an argument and returned a
+`delete_token` we could have all of our required information up-front and avoid the hassle of
+generating each part of this separately
+
+```sql
+CREATE TABLE aliases (
+    alias VARCHAR(30) PRIMARY KEY,
+    hash BYTEA NOT NULL REFERENCES hashes(hash) ON DELETE CASCADE,
+    delete_token VARCHAR(30) NOT NULL
+);
+```
+
+
+### SettingsRepo 
+This is used for generic server-level storage. The file & object stores keep their current path
+generator values here. This is also used in some migrations to mark completion.
+
+methods:
+- set
+- get
+- remove
+
+could be a simple table with String key & values
+
+pict-rs currently treats the value here as Bytes, so it could either be hex-encoded or translated to
+a string
+```sql
+CREATE TABLE settings (
+    key VARCHAR(80) PRIMARY KEY,
+    value VARCHAR(80) NOT NULL
+);
+```
+
+
+### IdentifierRepo
+Used to relate details (image metadata) to identifiers (image paths). Identifiers are currently
+treated as bytes, so may need hex-encoding to store in the database. They _should_ be valid strings
+in most environments, so it might be possible to drop the bytes requirement & instead have a string
+requirement.
+
+methods:
+- relate_details
+- details
+- cleanup
+
+```sql
+CREATE TABLE details (
+    identifier TEXT PRIMARY KEY,
+    details JSONB NOT NULL,
+);
+```
+
+
+### QueueRepo
+This is going to be the troublesome table. It represents jobs that will be processed. Jobs are
+pushed as Bytes, but at a higher level are actually JSON strings. The QueueRepo API could be updated
+to take `T: Serizlie` as input rather than bytes, and then we can store it as JSONB. With the
+current API, the repo doesn't need to know the shape of a job, and maybe that is a benefit. We
+should take care in the future not to query on the contents of the job.
+
+methods:
+- requeue_in_progress
+- push
+- pop
+
+```sql
+CREATE TYPE job_status AS ENUM ('new', 'running');
+
+CREATE TABLE queue (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    queue VARCHAR(30) NOT NULL,
+    job JSONB NOT NULL,
+    worker_id VARCHAR(30),
+    status job_status NOT NULL DEFAULT 'new',
+    queue_time TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX queue_worker_id_index ON queue INCLUDE worker_id;
+CREATE INDEX queue_status_index ON queue INCLUDE status;
+```
+
+claiming a job can be
+```sql
+DELETE FROM queue WHERE worker_id = '$WORKER_ID';
+
+UPDATE queue SET status = 'running', worker_id = '$WORKER_ID'
+WHERE id = (
+    SELECT id
+    FROM queue
+    WHERE status = 'new'
+    ORDER BY queue_time ASC
+    FOR UPDATE SKIP LOCKED
+    LIMIT 1
+)
+returning *;
+```
+
+
+### MigrationRepo
+This is used for migrating from local storage to object storage. It keeps track of which identifiers
+have been migrated, and on a successful migration, it is fully cleared.
+
+methods:
+- is_continuing_migration
+- mark_migrated
+- is_migrated
+- clear
+
+```sql
+CREATE TABLE migrations (
+    identifier TEXT PRIMARY KEY,
+);
+```
+
+
+### ProxyRepo
+This is used for keeping track of URLs that map to Aliases for proxied media.
+
+methods:
+- relate_url
+- related
+- remove_relation
+
+```sql
+CREATE TABLE proxies (
+    url PRIMARY KEY,
+    alias VARCHAR(30) NOT NULL REFERENCES aliases(alias)
+);
+```
+
+
+### AliasAccessRepo
+This is used for keeping track of aliases that are "cached" in pict-rs and can be safely removed
+when they are not useful to keep around. This might be able to piggyback on the aliases table or the
+proxies table.
+
+methods:
+- accessed
+- older_aliases
+- remove_access
+
+```sql
+ALTER TABLE aliases ADD COLUMN accessed TIMESTAMP;
+```
+or
+```sql
+ALTER TABLE proxies ADD COLUMN accessed TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP;
+```
+
+
+### VariantAccessRepo
+This is used for keeping track of access times for variants of an image to enable freeing up space
+from seldom-accessed variants. This might be able to piggyback on the variants table.
+
+methods:
+- accessed
+- contains_variant
+- older_variants
+- remove_access
+
+```sql
+ALTER TABLE variants ADD COLUMN accessed TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP;
+```