PostgreSQL is a highly robust relational database that offers extensive functionality for managing and manipulating data. However, handling deleted rows and implementing undo-redo operations can be challenging due to its Multi-Version Concurrency Control (MVCC) mechanism. In this guide, we will explore how to leverage PostgreSQL extensions pg_surgery and pageinspect to retrieve and manipulate dead tuples effectively. We will also discuss these extensions in detail.
What Are pg_surgery and pageinspect?
pg_surgery
The pg_surgery extension is a powerful tool that was introduced to assist in repairing and analyzing PostgreSQL databases. It allows database administrators to manipulate and manage corrupted or problematic tuples. This extension is particularly useful in situations where data recovery or forensic analysis is required, as it provides functions to directly modify tuple headers and restore deleted tuples. It includes the heap_force_freeze function, which can reset the visibility map bits of tuples to make them visible again, enabling restoration or further processing of deleted data. However, using this extension comes with risks and should be handled with care to avoid unintended data corruption.
page inspect
The pageinspect extension enables the inspection of PostgreSQL database pages at a low level. It provides functions to analyze the internal structure of pages and tuples, including their headers, visibility, and data contents. This extension is instrumental in understanding how data is stored and accessed, making it valuable for debugging, performance tuning, and forensic analysis.
Implementing Dead Tuple Retrieval and Manipulation
Retrieving and manipulating dead tuples involves restoring deleted rows (undo) and re-deleting restored rows (redo). These operations can be implemented using custom PostgreSQL functions and the aforementioned extensions.
Step 1: Enable Extensions
To begin, enable the pg_surgery and page inspect extensions in your database:
CREATE EXTENSION IF NOT EXISTS pg_surgery;
CREATE EXTENSION IF NOT EXISTS pageinspect;
Step 2: Create Helper Functions
1. Function to Get Deleted Records
This function retrieves information about deleted tuples from a specified table:
CREATE OR REPLACE FUNCTION get_deleted_records(table_name TEXT)
RETURNS TABLE (
id INTEGER,
ctid TID,
page_no INTEGER
) AS $$
DECLARE
v_max_page INTEGER;
BEGIN
-- Get the number of pages in the relation
EXECUTE format('
SELECT pg_relation_size(%L) / current_setting(''block_size'')::integer - 1;
', table_name) INTO v_max_page;
-- Return only truly deleted records from all pages
RETURN QUERY EXECUTE format('
WITH RECURSIVE page_data AS (
SELECT
get_byte(t_data, 0) +
(get_byte(t_data, 1) << 8) +
(get_byte(t_data, 2) << 16) +
(get_byte(t_data, 3) << 24) as record_id,
t_ctid,
page_no,
t_xmax,
t_xmin,
t_infomask,
t_infomask2
FROM generate_series(0, %s) as page_no,
LATERAL heap_page_items(get_raw_page(%L, page_no))
WHERE lp_len > 0 -- Ensure tuple is not completely dead
AND t_data IS NOT NULL
)
SELECT DISTINCT
record_id::integer as id,
t_ctid as ctid,
page_no
FROM page_data pd
WHERE t_xmax <> 0 -- Look for deleted tuples
AND NOT EXISTS (
-- Check if this record exists in the current table state
SELECT 1
FROM %I
WHERE ctid = pd.t_ctid
)
AND NOT EXISTS (
-- Check if this record was updated (rather than deleted)
SELECT 1
FROM %I
WHERE xmin = pd.t_xmax
)
ORDER BY page_no DESC, t_ctid DESC;
', v_max_page, table_name, table_name, table_name);
END;
$$ LANGUAGE plpgsql;
2. Function to Restore Deleted Rows (Undo)
This function restores a deleted row by reactivating its tuple:
CREATE OR REPLACE FUNCTION undo_delete(table_name TEXT, target_id INT)
RETURNS VOID AS $$
DECLARE
v_ctid TID;
v_exists BOOLEAN;
v_success INT;
v_payload TEXT;
BEGIN
-- Check if record already exists (already restored)
EXECUTE format('
SELECT EXISTS (
SELECT 1
FROM %I
WHERE id = %L
);
', table_name, target_id) INTO v_exists;
IF v_exists THEN
RAISE NOTICE 'Record with ID % is already restored in table %', target_id, table_name;
RETURN;
END IF;
-- Get the most recent deleted tuple's ctid
SELECT ctid INTO v_ctid
FROM get_deleted_records(table_name)
WHERE id = target_id
LIMIT 1;
IF v_ctid IS NULL THEN
RAISE EXCEPTION 'No recently deleted tuple found for ID % in table %', target_id, table_name;
END IF;
-- Restore the tuple
EXECUTE format('
SELECT COUNT(*) FROM heap_force_freeze(%L::regclass, ARRAY[%L]::tid[]) AS t(success);
', table_name, v_ctid) INTO v_success;
IF v_success > 0 THEN
-- Force system catalog update
EXECUTE format('ANALYZE %I;', table_name);
-- Create payload for notifications
v_payload := format('{"action": "restore", "id": %s}', target_id);
-- Notify system about the change using multiple notification channels
EXECUTE format('NOTIFY %I, %L', table_name, v_payload);
-- Send table_modified notification
v_payload := json_build_object(
'table', table_name,
'action', 'restore',
'id', target_id
)::text;
PERFORM pg_notify('table_modified', v_payload);
-- Send system_event notification
v_payload := json_build_object(
'type', 'row_restored',
'table', table_name,
'id', target_id
)::text;
PERFORM pg_notify('system_event', v_payload);
-- Force a table refresh
PERFORM pg_notify('table_refresh', table_name);
RAISE NOTICE 'Successfully restored deleted record with ID % in table %', target_id, table_name;
ELSE
RAISE EXCEPTION 'Failed to restore record with ID % in table %', target_id, table_name;
END IF;
END;
$$ LANGUAGE plpgsql;
ANALYZE: The ANALYZE command updates PostgreSQL's internal statistics about the table after restoring a tuple. This ensures the query planner recognizes the changes, optimizing subsequent queries involving the table.
3. Function to Redo Deletion
This function re-deletes a previously restored row:
CREATE OR REPLACE FUNCTION redo_delete(table_name TEXT, target_id INT)
RETURNS VOID AS $$
DECLARE
v_ctid TID;
v_page INT;
v_found BOOLEAN := FALSE;
v_exists BOOLEAN;
BEGIN
-- First check if the record exists in the table
EXECUTE format('
SELECT EXISTS (
SELECT 1
FROM %I
WHERE id = %L
);
', table_name, target_id) INTO v_exists;
-- If record doesn't exist, it means it's already deleted
IF NOT v_exists THEN
RAISE NOTICE 'Record with ID % is already deleted in table %', target_id, table_name;
RETURN;
END IF;
-- Initialize v_page to 0
v_page := 0;
-- Find the live tuple
LOOP
EXECUTE format('
SELECT t_ctid, TRUE
FROM heap_page_items(get_raw_page(%L, %s))
WHERE t_xmax = 0 -- Look for live tuples
AND lp_len > 0
AND t_data IS NOT NULL
AND get_byte(t_data, 0) + (get_byte(t_data, 1) << 8) +
(get_byte(t_data, 2) << 16) + (get_byte(t_data, 3) << 24) = %L
LIMIT 1;
', table_name, v_page, target_id)
INTO v_ctid, v_found;
-- Exit if found or if we've checked the first page
-- Most recently restored records should be in the first page
EXIT WHEN v_found OR v_page >= 0;
v_page := v_page + 1;
END LOOP;
IF NOT v_found THEN
RAISE EXCEPTION 'Could not locate the record with ID % in table % to redo deletion', target_id, table_name;
END IF;
-- Perform delete operation on the found tuple
EXECUTE format('
DELETE FROM %I WHERE ctid = %L;
', table_name, v_ctid);
RAISE NOTICE 'Successfully redone delete operation for ID % in table %', target_id, table_name;
END;
$$ LANGUAGE plpgsql;
Usage
* To restore a deleted row:
SELECT undo_delete('weather', 3); * To redo a deletion:
SELECT redo_delete('weather', 3);Considerations
1. Risk: Using pg_surgery can lead to data corruption if not handled properly. Always test in a safe environment.
2. Security: Restrict access to these functions to prevent misuse.
3. Audit: Log changes to maintain a record of operations.
By combining pg_surgery and pageinspect, PostgreSQL administrators can effectively retrieve and manipulate dead tuples, making data management more robust and versatile.