SQL: UPDATE index SET field = other_field 1

In an online shop, for prices we often use different currencies converted to a single currency for comparison. When the currency exchange rate changes, we want to run a statement as such (to update the EUR price converted to USD for all documents):

UPDATE index SET price_usd = price_in_original_currency * 1.25 WHERE currency = 'EUR';  

But we get the error:
syntax error, unexpected identifier near 'price_in_original_currency * 1.25'

So the feature request is:
For an UPDATE...SET query, please allow using the a field as the new value (with simple arithmetic expressions).

Start by reproducing the UPDATE index SET query shown in the issue and trace how UPDATE...SET values are parsed and evaluated. Identify the relevant SQL parser and update execution entry points in the repository; done means field references and simple arithmetic expressions work for the demonstrated currency conversion without breaking existing UPDATE statements.