chore: added a composite index to tx_output to reduce query time

[andreabadesso]

Motivation

Problem

The following query was taking ~90 seconds to execute:

UPDATE `address_balance`
SET `unlocked_authorities` = (
    SELECT BIT_OR(`authorities`)
    FROM `tx_output`
    WHERE `address` = 'HH5As5aLtzFkcbmbXZmE65wSd22GqPWq2T'
      AND `token_id` = '00'
      AND `locked` = FALSE
      AND `spent_by` IS NULL
      AND `voided` = FALSE
)
WHERE `address` = 'HH5As5aLtzFkcbmbXZmE65wSd22GqPWq2T'
  AND `token_id` = '00'

Root Cause

MySQL was choosing tx_output_spent_by_idx, scanning ~1M rows (all unspent UTXOs) and filtering by address/token_id afterward.

Solution

Add a composite covering index:

CREATE INDEX idx_tx_output_utxo_lookup 
ON tx_output (address, token_id, spent_by, voided, locked, authorities);

Results

Metric	Before	After
Index used	spent_by_idx	idx_tx_output_utxo_lookup
Rows scanned	1,002,812	270,830
Filtered	6.25%	100%
Extra	Using index condition; Using where	Using index (covering)

Query time expected to drop from ~90s to milliseconds.

Index disk usage

On the current database with ~7.1M rows, the new index idx_tx_output_utxo_lookup uses ~1.26 GB.
At 10x scale (~71M rows), estimated size: ~12.6 GB.

Acceptance Criteria

• We should check if the index already exist before creating it because the index was already created in mainnet-staging

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged
• Make sure either the unit tests and/or the QA tests are capable of testing the new features
• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.