An Error "UBTreeSearch::read_page has conflict with recovery, please try again later" Is Reported when a Service Uses a Standby Node to Read Data

Symptom

When the service uses the standby node to read data, an error (error code 43244) is reported. The error information contains "UBTreeSearch::read_page has conflict with recovery, please try again later."

Analysis

When parallel or serial replay is enabled (if the GUC parameters recovery_parse_workers and recovery_max_workers are both set to 1, serial replay is enabled; if recovery_parse_workers is set to 1 and recovery_max_workers is greater than 1, parallel replay is enabled): If the query thread of the standby node scans indexes, a read lock is added to the index page. Each time a tuple is scanned, the visibility is checked. If the transaction corresponding to the tuple is in the committing state, the visibility is checked after the transaction is committed. Transactions committed on the standby node depend on the log replay thread. During this process, the index page is modified. Therefore, a lock is required. The query thread releases the lock of the index page during waiting. Otherwise, the query thread waits for the replay thread to commit the transaction, and the replay thread waits for the query thread to release the lock.

This error occurs only when the same index page needs to be accessed during query and replay. When the query thread releases the lock and waits for the transaction to end, the accessed page is modified. Figure 1 shows the process.

• When scanning tuples in the committing state, the standby node needs to wait for transactions to be committed because the transaction committing sequence and log generation sequence may be out of order. For example, the transaction tx_1 on the primary node is committed earlier than transaction tx_2, the commit log of tx_1 on the standby node is replayed after the commit log of tx_2. According to the transaction committing sequence, tx_1 should be visible to tx_2. Therefore, you need to wait for the transaction to be committed.
• When the standby node scans the index page, it is found that the number of tuples (including dead tuples) on the page changes and cannot be retried. This is because the scanning may be forward or reverse scanning. For example, after the page is split, some tuples are moved to the right page. In the case of reverse scanning, even if the retry is performed, the tuples can only be read from the left, the correctness of the result cannot be ensured, and the split or insertion cannot be distinguished. Therefore, retry is not allowed.

Figure 1 Analysis

Solution

If an error is reported, you are advised to retry the query. In addition, you are advised to select index columns that are not frequently updated and use the soft deletion mode (physical deletion is performed during off-peak hours) to reduce the probability of this error.