skiDBy (skih-Dee-Bee)

WAL TO DO WAL NEEDS TO REDO AND UNDO! REDO BASED ON LSN AND UNDO UNCOMMITED TRANSACTIONS

• TRANSACTION MANAGER

• CREATE BASIC TRANSACTION MANAGER

• CREATE GLOBAL COUNTER FOR LSN

• HAVE BASIC LEVELS OF TRANSACTIONS, UNCOMMITTED AND COMMITTED

• BUFFER MANAGER

• CHANGE X_MIN AND X_MAX FOR TUPLE HEADERS TO MATCH THE TRANSACTION IDS

• FOR UPDATES, THE ORIGINAL PLACE MAY BE CHANGED CAUSE IT AN UPDATE IS LARGER SIZE THAN THE ORIGINAL, NEEDS A NEW TUPLE PLACE IN PAGE AND MARK OLD TUPLE FOR DELETION

• WAL IN MEMORY

• IS A CIRCULAR ARRAY TYPE DS

• encode before inserting here

• WAL DISK WRITING

• APPEND ONLY

• TYPES OF WAL RECORDS:

  • Insert Records (e.g., XLOG_HEAP_INSERT): When a new tuple is inserted into a table, PostgreSQL writes a WAL record that details the insertion. This record includes:

    The target heap page and tuple offset. The transaction ID (XID) that performed the insertion. The new tuple’s data (or enough of a “delta” to redo the insert). Example: A new employee record is inserted; the WAL record indicates which page was modified, the new row’s data, and that transaction 1005 inserted it.

  • Update Records (e.g., XLOG_HEAP_UPDATE or XLOG_HEAP_HOT_UPDATE): For updates, PostgreSQL logs a record that describes the change to an existing tuple:

    It may include both the old version (for undo purposes) and the new version. In the case of a “HOT” (Heap Only Tuple) update, the record indicates that the change does not require a change to associated index entries. Example: An employee’s salary is increased; the WAL record shows the before and after values and the location of the tuple on the heap page.

  • Delete Records (e.g., XLOG_HEAP_DELETE): When a tuple is deleted, the corresponding WAL record logs:

    The removal action, including the page and tuple position. The transaction ID responsible for the deletion. Example: An employee record is removed; the WAL record marks that tuple as deleted so that recovery can either redo or undo the deletion as needed.

  • Begin/Commit/Abort Records: Commit Record (e.g., XLOG_COMMIT): Indicates that a transaction’s changes are now durable. Abort Record: Marks a transaction as rolled back. These records ensure that during recovery, PostgreSQL can tell which transactions completed and which did not. Example: When a transaction that updated several rows commits, a commit record with its own LSN is written to confirm the transaction’s durability.

  • Full Page Image Records: Sometimes, rather than logging only the change (delta), PostgreSQL writes a full copy of a data page to the WAL. This happens when a page is modified for the first time after a checkpoint. Purpose: This ensures that during recovery the entire page can be restored if partial writes or torn pages occur. Example: A heavily modified table page may have its entire image logged so that recovery can reapply the whole page rather than piecing together multiple small changes.

  • B-Tree and Other Index Operation Records: Index modifications are separately logged. For instance:

    XLOG_BTREE_INSERT, XLOG_BTREE_DELETE, XLOG_BTREE_UPDATE: These records log changes in B-Tree index structures (which are common in PostgreSQL). They include information on the affected index pages and the keys or pointers changed. Example: When an index entry is added to support a new row, an index insert record is written to the WAL to record the new index structure.

- RECOVERY - so in the redo undo... all transactions commited and uncommited are applied if the page LSN is less than the transactions.... then all uncommitted transactions are undone after

• CHECKPOINTING
• WHEN DATABASE CASES IT WILL GO THROUGH EVERY PAGE. IF THAT PAGE LSN IS LESS THAN TRANSACTION LSN THEN APPLY EVERY LSN TRANSACTION UNTIL THE COUNTER IS CORRECT
• !!! NEED TO RELOOK AT HOW THE WAL IS CLEARED DURING HCECKPOINTING STILL DONT UNDERSTAND
• ALL I KINDA UNDERSTAND IS THAT ITS SO THE WAL DOESNT GROW INDEFINITELY AND CAN REUSE OLD LSNS BUT I DONT UNDERSTAND HOW THE PREVIOUS RECORDS LSNS ARE REUSED (LIKE SINCE THE LSN VALUE WRAPS AROUND WHEN IT GETS TO BIG)

- Buffer Tags - Buffer Descriptors

• Oversized tuples
• Ring buffer for big tables

other things

• Add custom wrror logic

TODO LATER

• SYSTEM CATALOG FOR WHERE FILES SHOULD GO
• BOOTLOADER
  • LOAD ALL INDEX PAGES TO MEMORY ON STARTUP
• VACCUM AND AUTO-VACCUUM
  • ID X_MAX IS SET FOR TUPLE, REMOVE DEAD TUPLES
  • only vaccuum commited transactions, so you can clean dead tuples from uncommited
• MORE COMPLEX TRANSACTIONS SYSTEM
  • Read Uncommitted: This is the lowest level of isolation where a transaction can see uncommitted changes made by other transactions. This can result in dirty reads, non-repeatable reads, and phantom reads.
  • Read Committed: In this isolation level, a transaction can only see changes made by other committed transactions. This eliminates dirty reads but can still result in non-repeatable reads and phantom reads.
  • Repeatable Read: This isolation level guarantees that a transaction will see the same data throughout its duration, even if other transactions commit changes to the data. However, phantom reads are still possible.
  • Serializable: This is the highest isolation level where a transaction is executed as if it were the only transaction in the system. All transactions must be executed sequentially, which ensures that there are no dirty reads, non-repeatable reads, or phantom reads.
• POST MASTER AND OTHER POSTGRES TINGS
• QUERY SYNTAX
• CONNECTION POOLER
• COMPOSITE KEYS
• Add Maddie somehow (skibity pookie)

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