[FIX] stock_valuation_layer: add an index to stock_valuation_layer_id field to improve performance

The index is being added because when processing a large PoS session, the method consume_specific_qty is called a large number of times if the product categories are set to automatic valuations. This method has the line:
tmp_value += qty_taken_on_candidate * ((candidate.value + sum(candidate.stock_valuation_layer_ids.mapped('value'))) / candidate.quantity)

This line, when running upwards of 20,000 times, can cause large performance issues, due to the stock_valuation_layer_ids being a related field on stock_valuation_layer_id.
This causes a doubling of the number of sql queries which causes slowdowns in performance. Adding this index drastically improves the performance of the queries generated by this line of code.

opw-3295560

closes odoo/odoo#126164

X-original-commit: 6f491e34
Signed-off-by: William Henrotin (whe) <whe@odoo.com>

addons/stock_account/models/stock_valuation_layer.py

@@ -26,7 +26,7 @@ class StockValuationLayer(models.Model):
     remaining_qty = fields.Float(readonly=True, digits='Product Unit of Measure')
     remaining_value = fields.Monetary('Remaining Value', readonly=True)
     description = fields.Char('Description', readonly=True)
-    stock_valuation_layer_id = fields.Many2one('stock.valuation.layer', 'Linked To', readonly=True, check_company=True)
+    stock_valuation_layer_id = fields.Many2one('stock.valuation.layer', 'Linked To', readonly=True, check_company=True, index=True)
     stock_valuation_layer_ids = fields.One2many('stock.valuation.layer', 'stock_valuation_layer_id')
     stock_move_id = fields.Many2one('stock.move', 'Stock Move', readonly=True, check_company=True, index=True)
     account_move_id = fields.Many2one('account.move', 'Journal Entry', readonly=True, check_company=True, index="btree_not_null")