Comprehensive and Detailed In-Depth Explanation:

Adding an index to a searched field in a MySQL table with over 100,000 rows improves query performance by reducing the number of rows scanned during searches, joins, or filters. The benefit of an index depends on the field’s data type, cardinality (uniqueness), and query patterns. MySQL indexing best practices, as aligned with Appian’s Database Optimization Guidelines, highlight scenarios where indices are most effective.

Option A (The field contains a textual short business code):This benefits greatly from an index. A short business code (e.g., a 5-10 character identifier like "CUST123") typically has high cardinality (many unique values) and is often used in WHERE clauses or joins. An index on this field speeds up exact-match queries (e.g., WHERE business_code = 'CUST123'), which are common in Appian applications for lookups or filtering.

Option C (The field contains many datetimes, covering a large range):This is highly beneficial. Datetime fields with a wide range (e.g., transaction timestamps over years) are frequently queried with range conditions (e.g., WHERE datetime BETWEEN '2024-01-01' AND '2025-01-01') or sorting (e.g., ORDER BY datetime). An index on this field optimizes these operations, especially in large tables, aligning with Appian’s recommendation to index time-based fields for performance.

Option D (The field contains big integers, above and below 0):This benefits significantly. Big integers (e.g., IDs or quantities) with a broad range and high cardinality are ideal for indexing. Queries like WHERE id > 1000 or WHERE quantity < 0 leverage the index for efficient range scans or equality checks, a common pattern in Appian data store queries.

Option B (The field contains long unstructured text such as a hash):This benefits less. Long unstructured text (e.g., a 128-character SHA hash) has high cardinality but is less efficient for indexing due to its size. MySQL indices on large text fields can slow down writes and consume significant storage, and full-text searches are better handled with specialized indices (e.g., FULLTEXT), not standard B-tree indices. Appian advises caution with indexing large text fields unless necessary.

Option E (The field contains a structured JSON):This is minimally beneficial with a standard index. MySQL supports JSON fields, but a regular index on the entire JSON column is inefficient for large datasets (>100k rows) due to its variable structure. Generated columns or specialized JSON indices (e.g., using JSON_EXTRACT) are required for targeted queries (e.g., WHERE JSON_EXTRACT(json_col, '$.key') = 'value'), but this requires additional setup beyond a simple index, reducing its immediate benefit.

For a table with over 100,000 rows, indices are most effective on fields with high selectivity and frequent query usage (e.g., short codes, datetimes, integers), making A, C, and D the optimal scenarios.