Databases &amp; Architecture

How MVCC enables non-blocking reads

Writers create new row versions instead of overwriting, readers see a consistent snapshot, so readers never block writers.

Two-Phase Locking and serializability

A growing phase only acquires locks, a shrinking phase only releases, no lock taken after one is freed.

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Write skew under snapshot isolation

Two transactions read an overlapping set, each writes disjoint rows, jointly violating an invariant.

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How the write-ahead log ensures atomicity and durability

Log changes before applying, flush log at commit, replay redo and undo on recovery.

What a database index is and when it helps

An index is a sorted lookup structure avoiding full scans, helps selective WHERE/JOIN columns, but costs write overhead.

Composite index column order for multi-column filters

Create one index on (last_name, first_name); order matters because the index serves leftmost-prefix lookups.

Clustered versus non-clustered indexes

A clustered index orders the table's actual rows, one per table; a non-clustered index is a separate structure pointing to rows.

What is a query execution plan?

The plan is the optimizer's chosen tree of operators; run EXPLAIN or EXPLAIN ANALYZE; watch for sequential scans, bad row estimates, and costly joins.

Trade-offs of adding indexes to a table

Indexes speed reads but slow writes since every INSERT, UPDATE, and DELETE must maintain them; they consume storage and can be unused on low-selectivity columns.

What is a covering index?

A covering index contains every column a query needs so the engine answers from the index alone, skipping the table heap; build it by including filter, join, and selected columns.

B-Tree versus Hash indexes

B-Trees keep keys sorted, supporting equality, range, prefix, and ORDER BY; hash indexes give O(1) equality only, no ranges or ordering.

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Optimizer picks nested loop over hash join

Nested loop wins on few rows, so bad row estimates from stale stats or skewed data trick it; fix by refreshing statistics, adding histograms, rewriting predicates, or ensuring memory for hashing.

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Indexing a low-cardinality status column

With three values each matches a third of rows, so the optimizer prefers a scan over costly random heap fetches; alternatives include partial indexes on rare values and composite indexes leading with status.

What is a database page?

A page is a fixed-size block, often 8KB, holding multiple rows; databases read and write whole pages because disk and OS I/O are block-oriented, amortizing seek cost and matching the buffer pool unit.

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Purpose of the Write-Ahead Log

WAL records changes sequentially and is flushed to disk before commit; the rule is log first, then data pages may lag; on crash the database replays the log to recover committed work.

Database checkpoints with WAL

A checkpoint flushes dirty pages and records a known-good point so recovery can start later in the log; frequent checkpoints shorten recovery but add I/O spikes, infrequent ones lengthen…

Heap file versus clustered index

A heap stores rows unordered with cheap inserts but no inherent ordering; a clustered or index-organized table stores rows in primary-key order, giving fast key range reads but costlier inserts and page…

Lifecycle of a single row update

Buffer manager faults the page in, the row is modified in memory marking the page dirty, a WAL record is written, and commit fsyncs the log while the dirty page is flushed later by a checkpoint.