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DBMS → Introduction to DBMS 38

Pinning a buffer page means
Marking it as do-not-evict (currently in use)
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Multi-level index builds
Index on the index to reduce levels needed to find record
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Selectivity of a predicate is
Fraction of tuples satisfying the predicate (between 0 and 1)
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Equivalence rules in query optimization allow
Transforming one RA expression to another producing same result
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The iterator (volcano) model implements pipelining using
Open(), GetNext(), Close() interface on each operator
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Materialization in query processing
Fully computes and stores intermediate result before passing to next operator
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External sort complexity for n blocks, B buffer frames is approximately
O(n log_B n) passes
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ISAM supports
Both sequential access AND indexed access (but static overflow)
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Histogram in query optimizer stores
Distribution of attribute values for better cardinality estimation
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Why is sequential disk access faster than random?
No seek time or rotational latency between consecutive blocks
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Index seek vs index scan: index SEEK
Traverses B+tree to find specific key (fast for selective queries)
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Fill factor in index creation specifies
Percentage of each index page to fill leaving space for future inserts
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Linear hashing
Splits buckets incrementally in linear order without sudden doubling
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Extendible hashing solves static hashing by
Dynamically doubling directory size and splitting buckets as needed
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Double buffering allows
I/O of next block to overlap with CPU processing of current block
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The Selinger optimizer uses
Dynamic programming to find optimal join ordering with estimated costs
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Laravel Eloquent is
ORM using Active Record pattern mapping tables to PHP classes
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php artisan make:model Question creates
New Eloquent model class in app/Models
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Eloquent assumes table name is
Plural snake_case of model (User→users, Question→questions)
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To override Eloquent table name
Set protected $table = 'custom_name'
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Question::all() returns
Collection of all Question model instances
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Question::find(1) returns
Question with PK=1 or NULL if not found
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findOrFail($id) when ID not found
Throws ModelNotFoundException
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Eager loading with('posts') prevents
N+1 query problem by loading related models in 2 queries instead of N+1
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Eloquent create() requires
$fillable array set on model (mass assignment protection)
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$question->save() performs
INSERT for new model or UPDATE for existing model
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Soft delete uses
deleted_at TIMESTAMP column (NULL=active, timestamp=deleted)
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withTrashed() includes
All records including soft-deleted ones
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belongsToMany() is used for
Many-to-many relationship requiring pivot table
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chunk(100, callback) in Eloquent
Processes records 100 at a time preventing memory exhaustion
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DB::table('questions')->get() returns
Plain PHP stdClass objects (not Eloquent model instances)
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DB::table('questions')->where('is_deleted',0)->count() returns
Integer count of non-deleted questions
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php artisan migrate:fresh
Drops ALL tables then re-runs all migrations
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Schema::create() in migration
Creates a new database table
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$table->id() creates
AUTO_INCREMENT UNSIGNED BIGINT primary key column named id
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$table->string('question') creates
VARCHAR(255) column named question
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$table->text('solution') creates
TEXT column for large text content
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$table->timestamps() adds
created_at and updated_at TIMESTAMP NULL columns
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DBMS → Joins 1

A left-deep join tree has
All inner (right) inputs as base tables
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DBMS → Relational Model 1

Primary key in MySQL InnoDB automatically creates
Clustered index
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