Q1. How do indexes work in MongoDB and what is a compound index?
An index is a B-tree structure storing a sorted subset of a collection's fields, letting the planner find documents without scanning everything. Without an index a query ...
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Q2. What type of language is SQL?
**Declarative (non-procedural)**. You state *what* result you want, not *how* to compute it. The database's **query optimizer** decides the "how" — which indexes to use, ...
Q3. What is the difference between a clustered and a non-clustered index?
A clustered index defines the physical order of rows in the table. Because the data is stored in that order, a table can have only one clustered index, usually the primar...
Q4. How would you analyse and optimise a slow query?
Query optimisation starts by measuring. `EXPLAIN` shows the plan; `EXPLAIN ANALYZE` runs it and reports actual rows and time. Things to look for: sequential scans on larg...
Q5. What does DROP TABLE do?
Permanently removes the table definition, all data, indexes, triggers, and constraints. **Cannot be undone** (unless in a transaction that rolls back, in some DBs). ```sq...
Q6. What are Indexes?
Data structures (typically B-Trees) that speed up data retrieval at the cost of slower writes and extra storage. Think of them as a book's table of contents.
Q7. Clustered vs Non-Clustered?
- **Clustered**: Determines physical storage order. One per table. The data IS the index. - **Non-Clustered**: Separate structure with pointers to data. Multiple allowed ...
Q8. Process of finding a good query execution strategy?
**Query optimization**. The database's query optimizer generates multiple execution plans and picks the cheapest one based on statistics, indexes, and cost models. You ca...