在数据库中批量写入 WAL 文件不会否定 WAL 文件的目的吗？

Question

I am reading about databases and I can't understand one thing about WAL files. They exist in order to make sure transactions are reliable and recoverable, however, apparently, to improve performance, WAL files are written in batches instead of immediately. This looks to me quite contradictory and negates the purpose of WAL files. What happens if there's a crash between WAL commits? How does this differ from not having the WAL at all and simply fsync'ing the database itself periodically?

Answer 1

I've no much idea and just seeked for information about this as it seems interesting to me.

If some ninja find my explanation incorrect please, correct me. What I understand at this point is that WAL files are written before the commit, then once confirmed that the transaction data is on the WAL, it confirms the transaction.

What is done in batch is to move this WAL data to heap and index, real tables.

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Write-Ahead Logging (WAL) is a standard method for ensuring data integrity. A detailed description can be found in most (if not all) books about transaction processing. Briefly, WAL's central concept is that changes to data files (where tables and indexes reside) must be written only after those changes have been logged, that is, after log records describing the changes have been flushed to permanent storage. If we follow this procedure, we do not need to flush data pages to disk on every transaction commit, because we know that in the event of a crash we will be able to recover the database using the log: any changes that have not been applied to the data pages can be redone from the log records. (This is roll-forward recovery, also known as REDO.)

https://www.postgresql.org/docs/current/wal-intro.html