refactor(iota-indexer): add db schema diagram in README.md (#1594)

* refactor(iota-indexer): add db schema diagram in README.md

* refactor(iota-indexer): revert to previous formatting

* refactor(iota-indexer): add margin to diagram

* refactor(iota-indexer): make ERD diagram more concise

* refactor(iota-indexer):  describe all db schema in a separate schema.md file

* refactor(iota-indexer): remove redundant tables from schema.md

* refactor(iota-indexer): fix spacing in schema.rs

* refactor(iota-indexer): from md files with drprint

* refactor(iota-indexer): use list instead of tables in schema.md

* refactor(iota-indexer): add index names in schema.md

crates/iota-indexer/README.md

@@ -13,6 +13,10 @@ Iota indexer is an off-fullnode service to serve data from Iota protocol, includ
 >   - Nodes expose the `NameServiceConfig` API, whereas indexer instances do not.
 >   - Indexer instance expose the `ExtendedApi`, but nodes do not.
 
+## Database Schema
+
+For more in depth information check the [Database Schema](./schema.md).
+
 ## Steps to run locally
 
 ### Prerequisites

crates/iota-indexer/schema.md

@@ -0,0 +1,95 @@
+# Database Schema
+
+The Indexer pulls checkpoint data from the full node and populates the tables shown in the ERD diagram:
+
+![Database Schema](./database_schema.svg)
+
+> **NOTE**
+>
+> Since all tables are populated from checkpoint data the entities are closely related, in the `SQL` files the foreign keys are not specified, a manual study of data is needed to understand all relations.
+>
+> Migrations are generated by diesel cli, the basic schema can be found [schema.rs](src/schema.rs).
+> For more in depth understanding of the database tables, go to [migrations](migrations) folder, in the contained `SQL` the indexes, partitions & constraints are declared.
+>
+> - Tables `objects_history` & `transactions` have partitions, each partition is created based on `checkpoint_sequence_number` (related form the `checkpoints` table) it goes from `0` to `MAXVALUE`
+> - `__diesel_schema_migrations` table is managed by `diesel` cli when applying migrations
+
+## Indexes
+
+### Table `checkpoints`
+
+| Index name         | Keys                   |
+| ------------------ | ---------------------- |
+| checkpoints_epoch  | epoch, sequence_number |
+| checkpoints_digest | checkpoint_digest      |
+
+### Table `events`
+
+| Index name                        | Keys                                                                    |
+| --------------------------------- | ----------------------------------------------------------------------- |
+| events_package                    | package, tx_sequence_number, event_sequence_number                      |
+| events_package_module             | package, module, tx_sequence_number, event_sequence_number              |
+| events_event_type                 | event_type, text_pattern_ops, tx_sequence_number, event_sequence_number |
+| events_checkpoint_sequence_number | checkpoint_sequence_number                                              |
+
+### Table `objects`
+
+| Index name                         | Keys                       | Condition                                                 |
+| ---------------------------------- | -------------------------- | --------------------------------------------------------- |
+| objects_owner                      | owner_type, owner_id       | WHERE owner_type BETWEEN 1 AND 2 AND owner_id IS NOT NULL |
+| objects_coin                       | owner_id, coin_type        | WHERE coin_type IS NOT NULL AND owner_type = 1            |
+| objects_checkpoint_sequence_number | checkpoint_sequence_number |                                                           |
+| objects_type                       | object_type                |                                                           |
+
+### Table `objects_snapshot`
+
+| Index name                                  | Keys                            | Condition                                                 |
+| ------------------------------------------- | ------------------------------- | --------------------------------------------------------- |
+| objects_snapshot_checkpoint_sequence_number | checkpoint_sequence_number      |                                                           |
+| objects_snapshot_coin                       | owner_id, coin_type, object_id  | WHERE coin_type IS NOT NULL AND owner_type = 1            |
+| objects_snapshot_type                       | object_type, object_id          |                                                           |
+| objects_snapshot_owner                      | owner_type, owner_id, object_id | WHERE owner_type BETWEEN 1 AND 2 AND owner_id IS NOT NULL |
+
+### Table `objects_history`
+
+| Index name            | Keys                                             | Condition                                                 |
+| --------------------- | ------------------------------------------------ | --------------------------------------------------------- |
+| objects_history_owner | checkpoint_sequence_number, owner_type, owner_id | WHERE owner_type BETWEEN 1 AND 2 AND owner_id IS NOT NULL |
+| objects_history_coin  | checkpoint_sequence_number, owner_id, coin_type  | WHERE coin_type IS NOT NULL AND owner_type = 1            |
+| objects_history_type  | checkpoint_sequence_number, object_type          |                                                           |
+
+### Table `transactions`
+
+| Index name                              | Keys                       | Condition                  |
+| --------------------------------------- | -------------------------- | -------------------------- |
+| transactions_transaction_digest         | transaction_digest         |                            |
+| transactions_checkpoint_sequence_number | checkpoint_sequence_number |                            |
+| transactions_transaction_kind           | transaction_kind           | WHERE transaction_kind = 1 |
+
+### Table `tx_calls`
+
+| Index name                  | Keys                                      |
+| --------------------------- | ----------------------------------------- |
+| tx_calls_module             | package, module, tx_sequence_number       |
+| tx_calls_func               | package, module, func, tx_sequence_number |
+| tx_calls_tx_sequence_number | tx_sequence_number                        |
+
+### Table `tx_senders`
+
+| Index name                          | Keys               | Condition |
+| ----------------------------------- | ------------------ | --------- |
+| tx_senders_tx_sequence_number_index | tx_sequence_number | ASC       |
+
+### Tables `tx_recipients`
+
+| Index name                             | Keys               | Condition |
+| -------------------------------------- | ------------------ | --------- |
+| tx_recipients_tx_sequence_number_index | tx_sequence_number | ASC       |
+
+## Partitions
+
+### Tables `transactions`, `objects_history`
+
+|            Keys            |             Condition             |
+| :------------------------: | :-------------------------------: |
+| checkpoint_sequence_number | FOR VALUES FROM (0) TO (MAXVALUE) |