Indexes, just like relational database!

Week 4 on M101J - Indexes

• Indexes take addtional space, but provide much faster data retrieval.
• Creating indexes
• .ensureIndex( )
• e.g. "db.students.ensureIndex( { student_id : 1 } );
• e.g. "db.students.ensureIndex( { student_id : 1, "class" : -1 } );
• Multi-key index
• index key can be array
• e.g. db.bbb.insert( { a: [1, 2, 3], b: 1 } );
• keys cannot both be arrays
• e.g. db.bbb.insert( { a: [1, 2, 3], b: [4, 5, 6] } );
• Unique index
• index key has to be unique, duplicate not allowed
• e.g. db.students.ensureIndex( { student_id: 1, name: 1 }, {unique: true} );
• Foreground vs background indexing
• Foreground: fast, but blocks writes. Suitable for DBA. Can lock a replica while it's being indexed.
• Background: slow (2~4x slower), concurrent with writes. Suitable for developers in a production setting.
• .explained( )
• Useful to examine a query to see if indexing is utlilized
• Important keys: "cursor" (did it use BtreeCursor?), "nscannedObjects" (how many objects actually queried?)
• .hint( )
• $natural: returns result in its natural order
• .hint( { $natural: 1 } ) will use BasicCursor instead of BTreeCursor