Write-Ahead Logging (WAL) - indepth understanding

 in ACID, Atomicity features: 

1) All queries in a transaction must succeed or nothing. 

2) If 1 query fails in a transaction, all successful queries before should get rollbacked. 

3) If DB goes down, all the successful queries should be rollbacked. --- how DB knows??

ChatGPT said:

Great question! The last point gets to the heart of how databases maintain ACID — particularly Atomicity — in crash scenarios.

---

🔁 Atomicity Recap:

"All or nothing." Either all changes in a transaction are committed, or none at all — even if the DB crashes.

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❓How does the DB know what to rollback if it crashes?

The answer lies in Write-Ahead Logging (WAL).

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💾 What is Write-Ahead Logging (WAL)?

• WAL is a durability and atomicity mechanism used by PostgreSQL (and many other DBs).

• Before any changes are made to the actual data files:

  1. The DB writes log entries (called WAL records) to a separate transaction log.

  2. These logs describe exactly what changes the transaction will make.

  3. The log is flushed to disk before the actual data changes are applied.

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🔁 What Happens During a Crash?

1. Suppose a transaction starts and:

   • It makes a few updates (these are only in memory or WAL).

   • It hasn’t yet committed.

   • Then the DB crashes (e.g., power failure).

2. On restart:

   • The DB replays the WAL.

   • If it sees a transaction without a COMMIT log entry, it rolls back any of its changes.

   • If it sees a COMMIT, it applies the changes.

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🔒 Atomicity Achieved:

Scenario	WAL Status	DB Action After Crash
Transaction has not committed	No COMMIT in WAL	Discard uncommitted changes
Transaction has committed	COMMIT found in WAL	Redo the changes if needed

This mechanism ensures:

• No partial transaction is ever visible.

• Even if changes made it to memory or temp storage, they won’t reach the final data files unless the whole transaction commits.

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🧠 In Summary:

• Databases know what to rollback after a crash thanks to WAL.

• WAL is persisted before data changes, allowing safe rollback or recovery.

• PostgreSQL, MySQL (InnoDB), Oracle, etc., all use some form of write-ahead or redo logging to maintain Atomicity.

Let me know if you'd like a visual diagram or example flow of WAL and crash recovery.

You said:

begin transaction INSERT INTO Customers (CustomerID, CustomerName, City) VALUES (1, 'John Doe', 'New York'); UPDATE Customers SET City = 'Los Angeles' WHERE CustomerID = 1; DB crashed before commit, does the WAL saves the INSERT and UPDATE statements as the user entered?

ChatGPT said:

Great follow-up! Let's break this down:

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🔄 Short Answer:

No, WAL does not save the original SQL statements like INSERT or UPDATE.
Instead, it saves low-level physical/logical changes to the database pages (called WAL records).

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📦 What Does WAL Actually Store?

When you run:

sql
BEGIN;
INSERT INTO Customers (CustomerID, CustomerName, City) VALUES (1, 'John Doe', 'New York');
UPDATE Customers SET City = 'Los Angeles' WHERE CustomerID = 1;
-- CRASH before COMMIT

The database does not store those SQL strings in WAL. Instead, it records:

1. The row being inserted (tuple contents).

2. The old and new values of the updated row.

3. The page ID where each row lives.

4. A transaction ID (XID) that owns these changes.

5. Other metadata (like timestamps, LSNs, etc.).

This is done in a compact binary format optimized for crash recovery and replication — not human readability.

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🧠 So, What Happens After the Crash?

1. PostgreSQL restarts and reads WAL.

2. It sees the INSERT and UPDATE were done by a transaction that never committed.

3. So it:

   • Does not replay those changes.

   • Discards them, ensuring Atomicity (all-or-nothing).

Even if those changes had made it to the shared memory (buffer cache), they are rolled back using the WAL + transaction status.

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✅ Why This Design?

Because:

• Replaying SQL is not reliable or efficient — imagine if the table structure changed!

• WAL is much faster and crash-safe.

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🔧 Analogy

Think of it like this:

Layer	Example
User SQL	INSERT INTO ...
Execution plan	Parsed and optimized operation
WAL record	"Page 1234, offset 5: insert this tuple"
Recovery uses	WAL, not SQL

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📘 Extra: Can I See the WAL?

You can’t read WAL files directly as plain SQL, but PostgreSQL tools like pg_waldump can show what kinds of changes are recorded.

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Summary:

✅ WAL saves binary change records, not the original SQL.
❌ No INSERT or UPDATE SQL text is stored.
🧠 This is what allows PostgreSQL to ensure atomic rollback after a crash.

Let me know if you’d like a deeper look into what a WAL entry looks like!

You said:

how DB knows that the INSERT and UPDATE statements were already executed from WAL and now it shoudln't replay ?

ChatGPT said:

Excellent question! Let's get into the mechanism that allows PostgreSQL (and other ACID-compliant DBs) to know which WAL changes to replay and which to ignore after a crash.

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💡 Core Idea

PostgreSQL associates every WAL change with a Transaction ID (XID).
Upon crash recovery, it checks whether each transaction committed before the crash.
If not → all its WAL records are ignored (rolled back logically).

---

🔍 Step-by-Step: What Happens

Suppose we run:

sql
BEGIN;
INSERT INTO Customers ...;  -- XID = 1234
UPDATE Customers ...;
-- DB crashes before COMMIT;

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1. 📝 WAL Records Are Written

• WAL contains:

  • WAL record: "XID 1234 inserted a row at page X, offset Y"

  • WAL record: "XID 1234 updated a row at page A, offset B"

• COMMIT not yet written, so the transaction is incomplete.

---

2. 💥 Crash Happens

• The database shuts down abruptly.

• Some WAL records (INSERT, UPDATE) were flushed to disk — but the COMMIT record was not.

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3. 🔁 On Restart: Recovery Begins

• PostgreSQL replays WAL records starting from the last checkpoint.

• For each record, it checks:

  • Which Transaction ID (XID) the record belongs to.

  • Whether that transaction committed or not.

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4. 🧠 How It Knows COMMIT Status?

• PostgreSQL stores transaction commit info in the pg_xact (aka pg_clog) file.

• If XID has no COMMIT record, it’s considered aborted.

---

5. 🚫 Result: Changes Ignored

• All WAL changes made by that XID are skipped.

• Any partial in-memory changes are discarded.

• The DB remains consistent and atomic.

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✅ Summary Table

WAL Record Type	What PostgreSQL Does During Recovery
COMMIT record	Replays all changes for that transaction
No COMMIT record	Skips (ignores) all changes from that XID

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🔐 What Guarantees This Works?

• WAL is always written before data pages (Write-Ahead)

• COMMIT is only considered valid once WAL is flushed to disk (fsync)

• On recovery:

  • WAL replay + XID status lookup in pg_xact guarantees correctness

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📘 Optional Deep Dive Tools

• pg_waldump – inspect WAL entries

• pg_stat_activity – view active XIDs

• pg_xact – internal file that tracks XID status (not user-visible)

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Let me know if you'd like to simulate this or see how this works with checkpoints or replication!

You said:

if WAL is written first, it might be on RAM, WAL file content would have not written into disk (WAL file), if DB crashes that causes WAL changes also goes off, isn't it ?

ChatGPT said:

Excellent observation — and you're absolutely right to be skeptical!

Let’s walk through this in detail, because your concern is valid but PostgreSQL handles it carefully to protect against exactly that risk.

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⚠️ Your Concern:

“If WAL is written first but still resides in RAM and the DB crashes, wouldn't we lose WAL too?”

Yes — if WAL only lived in RAM, all atomicity and durability guarantees would be lost in a crash.

But PostgreSQL is designed to ensure WAL is flushed to disk safely before considering any transaction durable.

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✅ How PostgreSQL Handles This (Safely):

🔁 WAL Write = Two Steps:

1. Write to OS buffer (write() or writev() system call)

   • This goes to the OS page cache (RAM).

2. Flush to disk using fsync()

   • This forces the OS to physically persist WAL contents to disk (SSD/HDD).

   • This is the key durability step.

---

📦 COMMIT Only Happens After WAL Is Flushed

When a transaction issues COMMIT, PostgreSQL:

1. Writes a COMMIT WAL record to the WAL buffer.

2. Calls fsync() on the WAL file.

3. Only after fsync() successfully returns, it sends "COMMIT OK" to the client.

So if the DB crashes before fsync(), WAL is lost, and PostgreSQL knows to treat the transaction as never committed.

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🧠 Summary: Crash Safety Logic

Step	Description
WAL written (not flushed)	Still volatile — not durable yet
WAL flushed with fsync()	Now durable — safe from crash
Crash before fsync()	WAL lost → TX will be rolled back
Crash after fsync()	WAL survives → TX can be recovered

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📘 Configuration Option: synchronous_commit

• By default, PostgreSQL uses synchronous_commit = on

  • Forces WAL flush to disk before confirming commit.

• You can set synchronous_commit = off for speed, but then:

  • You risk losing committed transactions if the DB crashes before flushing WAL.

⚠️ Use synchronous_commit = off only if you can tolerate data loss.

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✅ Final Answer

WAL is only considered safe after being fsynced to disk.
PostgreSQL will not confirm a COMMIT to the client until WAL is flushed to disk.
This guarantees atomicity and durability, even in crashes.

Let me know if you'd like to see how this looks in logs or simulate it!

You said:

you have gone off topic, my question is simple, i haven't executed the commit yet. I just executed INSERT and UPDATE, these 2 transactions would be sitting in WAL, in RAM? now what will happen if DB crashes, doesn't WAL lose these 2 statements ?

ChatGPT said:

Ah — thanks for the clarification! You're absolutely right to focus on the pre-COMMIT phase, and you're asking a very precise question:

If I only execute INSERT and UPDATE (no COMMIT yet), are those WAL entries just sitting in RAM? If the DB crashes now, won’t I lose them?

---

✅ Short Answer:

No, you won’t lose them — because PostgreSQL flushes WAL to disk even before COMMIT in many cases.

Let’s go deeper.

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🔍 What Really Happens When You Run INSERT and UPDATE (Before COMMIT)

1. PostgreSQL generates WAL records for the INSERT and UPDATE.

2. These WAL records are:

   • First written into WAL buffers in RAM.

   • Then, periodically flushed to disk, even before COMMIT.

This flushing can happen due to:

• WAL buffer getting full.

• Background processes like wal_writer.

• checkpoint_timeout triggers.

• Explicit flushing (e.g. SELECT pg_switch_wal()).

So even before COMMIT, the WAL records might already be on disk.

---

⚠️ But What If DB Crashes Before WAL Flush?

If PostgreSQL crashes before those WAL entries are flushed from RAM to disk (via fsync()), then yes:

You will lose those uncommitted changes, because WAL was never persisted.

But That’s OK — It’s by Design.

• Those changes were part of an uncommitted transaction.

• Atomicity requires that they be discarded anyway.

• So PostgreSQL just forgets that XID during crash recovery — as if the INSERT and UPDATE never happened.

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🔁 Real-Life Timeline Example

Time	Action	WAL Status	Risk if Crash Now?
T1	BEGIN	No WAL yet	No risk
T2	INSERT, UPDATE	WAL in RAM only	WAL might be lost; TX uncommitted, so it's OK
T3	WAL buffer flushed to disk	WAL on disk	Changes still uncommitted — ignored at recovery
T4	COMMIT	COMMIT record written + WAL fsync	Safe — durable
T5	Crash	WAL replayed	Depends on last flushed point

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🧠 Summary

• Yes, WAL initially sits in RAM after INSERT/UPDATE.

• If not yet flushed, a crash will lose that WAL.

• But that’s fine — PostgreSQL will discard the entire uncommitted transaction anyway.

• WAL doesn't need to survive unless COMMIT was issued.

🔐 Atomicity is preserved: If you didn’t COMMIT, it’s like it never happened — even if WAL was lost in RAM.

---

Let me know if you want to simulate this or inspect pg_waldump for such uncommitted records!

You said:

what you mean by WAL on disk ? does that mean WAL file will be saved to disk ? or WAL file insturctions are executed onto DB on disk ?

ChatGPT said:

Excellent precision — you're asking exactly the right kind of question.

Let me explain this clearly and carefully.

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❓ What Does “WAL on Disk” Mean?

When we say “WAL is on disk”, it means:

✅ The WAL records (write-ahead log entries) have been physically flushed (via fsync()) from RAM to the WAL log files on disk — but they have NOT YET been applied to the actual database tables.

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🔁 Two Important Filesystems in PostgreSQL

File Type	What It Contains	Purpose
📝 WAL file (pg_wal/)	Logs of what changes will happen	Used for crash recovery and replication
📊 Data files (base/)	Actual table and index pages	The real state of the database

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🧠 What Actually Happens

Let's walk through the flow:

1. You run:

   sql
   BEGIN;
   INSERT INTO Customers ...
   UPDATE Customers ...
   

2. PostgreSQL:

   • Writes the changes to WAL buffer (RAM) as WAL records.

   • Modifies the shared buffers (in-memory versions of data pages).

   • Does not write to table files on disk yet.

3. At some point (before or at COMMIT):

   • WAL is flushed to disk into a .wal file in pg_wal/ folder using fsync().

4. Now:

   • WAL is on disk ✅

   • Actual DB files are still not updated on disk ❌ (changes live in shared buffer cache)

---

🔥 Crash Happens Now?

• PostgreSQL replays the WAL file on restart.

• It uses those WAL records to bring the database to a consistent state:

  • Re-applies changes to the data files from the WAL.

  • Only for transactions that had committed.

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❌ What It Does Not Mean:

❌ "WAL instructions are executed onto DB files on disk" when WAL is flushed

No — flushing WAL to disk does NOT mean changes are written to the actual tables on disk.
The actual DB pages may still be in memory (shared buffer pool).

They’ll be flushed to disk eventually, during:

• A checkpoint

• A buffer eviction

• Or manually via CHECKPOINT or pg_basebackup

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✅ Summary

Phrase	Meaning
"WAL is on disk"	WAL records were flushed (fsync) into .wal files in pg_wal/
"WAL instructions executed"	Happens later, during crash recovery or checkpoints
"Data is on disk"	Modified table/index pages have been flushed from shared buffers to disk

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Let me know if you'd like to see how WAL files look, or explore the timeline from INSERT to disk write in more detail.

You said:

so, when exactly WAL file content is written to Data files, is it after COMMIT ? because between INSERT and UPDATE, if i execute SELECT query on the INSERTED customer, I would get the results, now my question is how it's getting the results, if WAL instructgions are not yet executed to Data files ?

ChatGPT said:

Brilliant question — you’ve now stepped into the core of PostgreSQL’s MVCC architecture and buffer management.

You're absolutely right to ask:

“If WAL isn’t applied to the actual data files yet, how is SELECT after INSERT able to return the newly inserted data before COMMIT?”

Let’s go step by step.

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✅ Short Answer

PostgreSQL does not need to apply WAL to data files for a SELECT to see inserted/updated data.

That's because all changes happen in shared buffers (memory) immediately — not on disk.

---

🧠 Key Concepts

Term	Meaning
WAL	Write-ahead log — durable journal of changes (used for recovery)
Data Files	Persistent storage of table/index pages
Shared Buffers	In-memory cache of data pages, where all changes are made first
MVCC	Multi-Version Concurrency Control — enables per-transaction visibility

---

🏗️ What Happens When You INSERT, UPDATE, or DELETE?

Step-by-step (Before COMMIT):

1. You run:

   sql
   BEGIN;
   INSERT INTO Customers VALUES (1, 'John Doe', 'New York');
   

2. PostgreSQL:

   • Allocates a new tuple in a data page held in shared buffer (RAM).

   • Writes the row there (with your transaction's XID).

   • Generates a WAL record and stores it in WAL buffer (RAM).

   • Nothing written to disk (yet).

3. You run:

   sql
   SELECT * FROM Customers WHERE CustomerID = 1;
   

4. PostgreSQL:

   • Scans the shared buffer.

   • Finds the tuple you inserted.

   • Checks your transaction ID (XID) and visibility rules via MVCC.

   • ✅ Returns the row to you because it's your own uncommitted transaction.

---

📌 Important: Shared Buffers Are the Source of Truth (While in Memory)

Even though the WAL and actual data files are not yet updated on disk, the in-memory version of the data (shared buffer) contains:

• The inserted/updated data

• The row version information

• The visibility metadata (XID, xmin, xmax)

That’s all PostgreSQL needs to answer your SELECT.

---

💾 When Does WAL Content Get Written to Data Files?

The actual disk write of table/index files (i.e., applying WAL to data files) happens eventually, via:

1. Checkpoints (periodic background flushes)

2. Buffer eviction (when buffer cache is full)

3. Manual CHECKPOINT

4. Crash recovery, if crash happened before flushing

So:

• WAL is the durability log.

• Shared buffers are the working memory.

• Data files are not updated immediately after COMMIT — they lag behind.

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📘 Example Timeline

Time	Action	WAL on Disk?	Data Files Updated?	Can SELECT See It?
T1	BEGIN	❌	❌	✅ (transactional)
T2	INSERT, UPDATE	Maybe	❌	✅ (via MVCC)
T3	SELECT	N/A	❌	✅
T4	COMMIT	✅	❌	✅ (now visible to others)
T5	CHECKPOINT	✅	✅	✅

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✅ Final Summary

• SELECT sees your changes from shared memory, not from disk.

• WAL is not applied to data files immediately; it's used only for crash recovery.

• Data files are updated later (lazy flush via checkpoints).

• MVCC + shared buffers = PostgreSQL can show you uncommitted data instantly and safely.

---

Let me know if you’d like to walk through this with a pg_buffercache example or WAL replay!

You said:

how it's visible to others when the data is not yet written in Data Files (actual tables)

ChatGPT said:

Fantastic — you’ve zeroed in on one of the most misunderstood but core strengths of PostgreSQL's architecture:

How can other transactions see committed data if it hasn’t been written to disk yet?

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✅ Short Answer:

Other sessions (transactions) don’t read from disk — they read from shared memory (shared buffers) just like you do.

So once your transaction commits, your changes become visible to other transactions via the shared buffer, even if the actual table files on disk haven’t been updated yet.

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🧠 Let's Break It Down

🔧 PostgreSQL Architecture Recap

Component	Description
Shared Buffers	Main in-memory cache of table/index pages. All reads/writes happen here.
WAL	Crash-safe log, written before data file changes
Data Files	The actual .rel files on disk, updated lazily via checkpoints

---

🏗️ Timeline Example

Session A:

sql
BEGIN;
INSERT INTO users VALUES (1, 'Alice');
COMMIT;

• PostgreSQL:

  • Writes the row to shared buffer (memory).

  • Logs the change in WAL (maybe still in RAM or flushed to disk).

  • Doesn’t update the users data file on disk immediately.

  • Marks your XID as committed in pg_xact.

  • ✅ Now your row is visible to other transactions.

Session B (a moment later):

sql
SELECT * FROM users;

• PostgreSQL:

  • Looks in the shared buffers (where the committed row exists).

  • Checks MVCC visibility rules:

    • The inserting transaction's XID is marked as committed → ✅ visible.

  • ✅ Returns Alice’s row, even though the disk file hasn't been updated yet.

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🔍 Why It Works

1. All sessions share the same in-memory buffer pool (shared_buffers).

2. PostgreSQL’s MVCC engine uses transaction IDs (XID, xmin, xmax) to control visibility.

3. As long as your transaction is committed, and the row is in memory, it’s immediately visible to others.

---

💾 But What About Disk?

• The actual users table file on disk may still contain the old version of the page.

• But that’s okay — PostgreSQL:

  • Will write the in-memory page to disk eventually (e.g., on checkpoint).

  • On crash, it will use WAL to replay the committed changes into the table files.

---

🧪 Visualization

pgsql
[Session A]
 INSERT INTO users (1, 'Alice')
 └──> [Shared Buffers] ← immediate write
      └──> [WAL log]    ← write-ahead record

 COMMIT
 └──> XID marked as committed (pg_xact)

[Session B]
 SELECT * FROM users
 └──> Reads from shared buffers
     └──> Sees Alice’s row (XID committed)

[Disk Data Files]
 └──> May be updated later (not yet needed)

---

🧠 Summary

Question	Answer
Is the row written to disk before it's visible to others?	❌ No — it's visible via shared memory
How do other sessions see the row?	By reading the shared buffer and checking committed XID
When is the actual table file updated?	Later — during a checkpoint or flush
Is data safe if crash happens?	✅ Yes, as long as WAL was flushed to disk before COMMIT

---

Let me know if you want to try this in a demo with pg_buffercache or trace how XID changes with visibility!

You said:

so until XID is generated and marrked that record with XID, the data will be never saved into actual DAta files ?

ChatGPT said:

Fantastic question again — you're getting very close to exactly how PostgreSQL ensures atomicity, consistency, and crash safety.

Let’s clear it up:

---

✅ Short Answer:

Correct. Until a row is assigned an XID and that XID is considered committed, PostgreSQL will not write the change to actual data files (disk).

But let's clarify some nuances around:

• When XID is assigned

• What happens before/after COMMIT

• When data gets written to data files

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🧠 Step-by-Step Breakdown

🔹 1. When is XID Assigned?

• A transaction gets an XID when it performs its first write operation (e.g., INSERT, UPDATE, or DELETE).

• This XID is used to tag every tuple (row) the transaction modifies.

  • Stored in xmin (row creation XID) or xmax (row deletion XID).

🔹 2. Where is the Modified Row Written First?

• It’s written into a shared buffer (RAM) — the in-memory page cache.

• Not to disk yet.

• The WAL is generated at this point as well (write-ahead log entry created).

• The actual data file on disk is still untouched.

🔹 3. What Happens at COMMIT?

• PostgreSQL:

  • Writes a COMMIT WAL record.

  • Flushes the WAL to disk (fsync()).

  • Marks the XID as committed in pg_xact (commit log).

🔹 4. When Is the Row Written to the Actual Table File on Disk?

• Eventually:

  • When a checkpoint occurs, or

  • When that page in shared buffer is evicted, or

  • If you manually issue CHECKPOINT

Only then:

• The page is written to disk with that XID-marked row.

• PostgreSQL uses the committed XID to determine the row is valid.

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🧾 Visualization

sql
INSERT INTO employees ...

[XID Assigned]  ---> 1234
[Row Created in Shared Buffers]  ---> With xmin = 1234
[WAL Generated] ---> INSERT record logged
[COMMIT Issued] ---> COMMIT record written to WAL + XID marked committed
[Disk Files]    ---> Not updated yet
[Checkpoint Happens Later] ---> Modified page flushed to disk with XID=1234

---

📌 So Your Statement:

"Until XID is generated and marked, that record will never be saved to actual data files?"

✅ Mostly correct, with clarification:

• XID must be generated for the row to even exist.

• XID must be marked as committed before the page is ever written to disk.

• PostgreSQL delays writing to data files until it knows the transaction has committed and WAL is durable.

---

🧠 Why This Matters

• Prevents partial writes.

• Ensures crash recovery only replays safe, committed changes.

• Data files remain always consistent — thanks to WAL + buffer + XID coordination.

---

Let me know if you’d like to explore pg_xact, page inspection, or real example of WAL vs heap file write timing!

You said:

ok, let me ask you one more scenario, what if, while writing from WAL disk file to actual DB records, after INSERT it crashed, upon DB restart how it knows that INSERT is already executed but not UPDATE statement yet ?

ChatGPT said:

Brilliant scenario — this is exactly the kind of edge case PostgreSQL's crash recovery is designed to handle. Let's analyze it carefully:

---

🔍 Scenario Recap

You're describing:

1. A transaction does:

   sql
   BEGIN;
   INSERT ...;
   UPDATE ...;
   COMMIT;
   

2. The COMMIT has already flushed WAL to disk → ✅ durable.

3. But during the actual writing of WAL contents into the data files (table files) — maybe:

   • INSERT was written

   • UPDATE was not

   • And then, 💥 crash happens

Now the question is:
How does PostgreSQL know it needs to apply UPDATE but skip reapplying INSERT?

---

✅ Short Answer:

PostgreSQL replays WAL from the last checkpoint — and WAL is idempotent.
It does not care which changes were already partially written to disk.

It just re-applies both the INSERT and the UPDATE, using the WAL.
If a change (e.g., INSERT) was already written to disk before the crash, it's harmless to replay it again — because PostgreSQL writes to specific page offsets, not via SQL.

---

🧠 Why This Works: Idempotent WAL Replay

🔁 WAL Contains Low-Level, Page-Oriented Instructions:

Not:

sql
INSERT INTO ...;
UPDATE ...;

But something like:

"In page 104, at offset 3, insert this tuple with XID 1234"
"In page 105, at offset 7, replace this tuple with new version"

So during crash recovery:

• PostgreSQL replays all WAL entries after the last checkpoint.

• For every committed transaction, it applies all WAL entries again — regardless of whether they were partially written before.

✨ Safe Reapplication

• Reapplying INSERT: Already exists? Fine — it gets overwritten identically.

• Reapplying UPDATE: Maybe wasn't written yet — now it is.

This is crash-safe, atomic, and deterministic.

---

📁 Data Files Don't Track Partial WAL Execution

• PostgreSQL doesn't track whether INSERT was applied but UPDATE wasn't.

• Instead, it says:

  "If COMMIT is in WAL, I must replay everything for that XID. Doesn't matter what made it to disk before."

---

📌 Key PostgreSQL Mechanism

Mechanism	Role
WAL	Durable log of all changes before applying to data files
WAL Replay	Applies all changes for committed transactions during recovery
Idempotence	Safe to reapply same WAL entries (overwrite existing data safely)
Crash Recovery	Doesn't check what was already written — it replays WAL blindly

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✅ Final Summary

• WAL is the source of truth — not what's half-written in data files.

• PostgreSQL doesn't need to track partial writes to data files.

• WAL replay is idempotent: safe to reapply even if part of it already made it to disk.

• As long as COMMIT is in WAL, PostgreSQL knows:

  "Replay the whole transaction's WAL records — it's safe and correct."

---

Let me know if you want to explore how this relates to LSNs (Log Sequence Numbers), or how checkpoints optimize replay time!

You said:

ok, now assume, BEGIN; INSERT ...; UPDATE ...; COMMIT; all these are applied to actual tables successfully, now does these records gets removed from WAL ?

ChatGPT said:

Perfect — this is the right next question, and the answer shows the clever efficiency of PostgreSQL.

You're asking:

Once a transaction (e.g., BEGIN; INSERT; UPDATE; COMMIT;) is fully applied to actual table files (on disk),
✅ Do the corresponding WAL records get deleted?

---

✅ Short Answer:

No, WAL records are not immediately deleted after changes are applied to the data files.
Instead, PostgreSQL retains them until they are no longer needed for:

• Crash recovery

• Replication

• Archiving / backup

Once all those dependencies are cleared, old WAL segments are recycled or removed.

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🧠 Deeper Explanation

🔹 WAL Is Append-Only (Initially)

• All changes are logged into WAL files under pg_wal/ (formerly pg_xlog/).

• Even after the corresponding table pages are updated on disk, the WAL entries still exist.

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🔹 So When Can PostgreSQL Delete Old WAL?

Only after:

1. ✅ All data changes from those WAL records are flushed to disk (via checkpoint), and

2. ✅ No replication standby needs those WAL records, and

3. ✅ No backup process (like pg_basebackup or PITR) requires them

Then:

• Those WAL files are either deleted or recycled for reuse.

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📁 WAL Storage Lifecycle

Stage	WAL Still Kept?	Why?
Changes in shared buffers	✅ Yes	Not yet written to disk
Changes flushed to table files	✅ Yes	Still needed for crash recovery
Checkpoint completed	✅ Possibly	Still might be needed for replication or PITR
Replication lag caught up	❌ No	Safe to remove or recycle

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🔄 How PostgreSQL Cleans Up Old WAL

• Checkpoints mark which WAL entries are no longer needed for crash recovery.

• Replication slots track what standbys have replayed — oldest one holds back WAL.

• pg_wal directory is cleaned up by:

  • Background writer

  • WAL archiver (if enabled)

  • wal_keep_size and max_wal_size parameters

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🔧 Tools to Control WAL Cleanup

• wal_keep_size — how much WAL to keep for standbys

• archive_mode = on — keeps WAL for backups (until archived)

• pg_archivecleanup — deletes archived WAL

• pg_wal_lsn_diff() — see how much WAL has accumulated

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✅ Final Summary

Question	Answer
Are WAL records removed after COMMIT?	❌ Not immediately
When are they removed?	After checkpoint, replication, backup
Why are they kept?	For crash recovery, standby sync, PITR
Who removes them?	Background processes (and manually if needed)

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Let me know if you want to try querying WAL file size, checkpoint status, or LSN gaps between primary and standby!