pg_replication_slot_advance
`pg_replication_slot_advance()` is a PostgreSQL system function used to **manually advance the position of a replication slot** without consuming or streaming changes. This is useful for skipping unwanted data or moving a slot forward to a specific LSN (Log Sequence Number).
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## **Purpose**
– Move a replication slot’s confirmed flush LSN (`confirmed_flush_lsn`) to a specified position.
– This is helpful when you want to:
– Skip a large transaction or problematic data.
– Free up WAL files that are being retained because the slot is lagging.
– Manually manage replication slot positions in edge cases.
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## **Syntax**
“`sql
pg_replication_slot_advance(slot_name text, upto_lsn pg_lsn)
“`
– **slot_name**: Name of the replication slot.
– **upto_lsn**: The target LSN (inclusive) to advance to.
– **Returns**: A record with `slot_name` and `end_lsn` (the new position).
—
## **Important Notes**
1. **Requires superuser or replication role privileges**.
2. The slot can only be advanced **forward** (to a higher LSN).
3. The `upto_lsn` must be a valid LSN that exists in the WAL (or between current slot position and latest WAL).
4. If you advance past unconsumed changes, those changes will **never be consumed** by the slot’s downstream client.
5. This does **not** affect the physical replication slot on a standby (if it’s a physical slot), but works for logical slots.
—
## **Example Usage**
### 1. Check current slot position:
“`sql
SELECT slot_name, confirmed_flush_lsn
FROM pg_replication_slots
WHERE slot_name = ‘my_logical_slot’;
“`
### 2. Advance the slot to a specific LSN:
“`sql
SELECT * FROM pg_replication_slot_advance(‘my_logical_slot’, ‘0/3000000’);
“`
Output:
“`
slot_name | end_lsn
—————-+————
my_logical_slot | 0/3000000
“`
### 3. Verify the new position:
“`sql
SELECT slot_name, confirmed_flush_lsn
FROM pg_replication_slots
WHERE slot_name = ‘my_logical_slot’;
“`
—
## **Use Case Example**
Imagine a logical replication slot is stuck because a downstream subscriber cannot process a large, unwanted transaction. Instead of waiting or re-creating the slot, you can:
1. Find the LSN after the problematic transaction (e.g., from `pg_logical_slot_get_changes` with `upto_lsn`).
2. Advance the slot past that LSN:
“`sql
SELECT pg_replication_slot_advance(‘my_slot’, ‘0/5000000’);
“`
3. The slot now ignores skipped changes, and WAL retention moves forward.
—
## **Cautions**
– **Data loss for downstream**: Advanced changes are discarded for that slot.
– **Physical slots**: Works but be careful on standbys (may affect WAL retention).
– **Logical slots**: Ensure the subscriber can handle the skipped data (may cause inconsistencies).
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## **Related Functions**
– `pg_replication_slot_create()`: Create a slot.
– `pg_drop_replication_slot()`: Remove a slot.
– `pg_logical_slot_get_changes()`: Consume changes from a logical slot.
– `pg_current_wal_lsn()`: Get current WAL LSN.
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Let me know if you need help with a specific scenario or troubleshooting!
