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Elasticsearch Mapping: text vs keyword, Multi-Fields, Dynamic Mapping, nested and Why Mappings Can't Change

In Elasticsearch the mapping sets each field's type and how it is indexed; leave it to guessing and an ISBN becomes a number, a decimal price an integer, and you get wrong results without an error, so a production index needs dynamic: strict and a hand-written mapping. With real output: dynamic mapping traps, text vs keyword, title.keyword and title.folded multi-fields, scaled_float prices, object vs nested with a nested query in .NET, flattened, immutable mappings and the field limit.