The purpose of this document is to let users know how they can use zheap (a new storage format for PostgreSQL) and the work that is still pending. This new storage format provides a better control over bloat, reduces the tuple size and reduces the write amplification. The detail design of zheap is present in zheap design document (src/backend/access/zheap/README).
How do I use zheap?
We have provided a storage engine option which you can set when creating a table. For example:
create table t_zheap(c1 int, c2 varchar) with (storage_engine='zheap');
Index creation for zheap tables doesn't need any special syntax.
You can also set the GUC parameter storage_engine. The default value is “heap", but you can set it to “zheap”. If you do, all subsequently-created tables will use zheap.
These interfaces will probably change once the storage format API work is integrated into PostgreSQL. We’ll adjust this code to use whatever interfaces are agreed by the PostgreSQL community.
We have also provided a GUC called data_alignment, which sets the alignment used for zheap tuples. 0 indicates no alignment, 4 uses a maximum of 4 byte alignment, and any other value indicates align as per attalign. This also controls the padding between tuples. This parameter is just for some experiments to see the impact of alignment on database size. This parameter will be removed later; we’ll align as described in the zheap design document.
Each zheap page has fixed set of transaction slots each of which contains the transaction information (transaction id and epoch) and the latest undo record pointer for that transaction. By default, we have four transaction slots per page, but this can be changed by setting --with-trans_slots_per_zheap_page=value while configuring zheap.
What doesn’t work yet?
- Logical decoding
- Snapshot too old - We might want to implement this after first version is committed as this will work differently for zheap.
- Alter Table <table_name> Set Tablesapce <tbs_name> - For this feature to work correctly in zheap, while copying pages, we need to ensure that pending aborts gets applied before copying the page.
- pg_undo_dump similar to pg_wal_dump: We would like to develop this utility as it can be used to view undo record contents and can help us debug problems related to undo chains.
- We also want to develop tools like pgstattuple, pgrowlocks that allow us to inspect the contents of database pages at a low level.
- wal consistency checker: This will be used to check for bugs in the WAL redo routines. Currently, it is quite similar to what we have in current heap, but we want to extend it to check the consistency of undo pages similar to how it checks for data and index pages.
- Currently, the TPD pages are not added to FSM even if they can be completely reused.
- Single user mode: This needs some investigation as to what exactly is required. I think we need to ensure that undo gets applied without the need to invoke undo worker.
The other pending code related items are tracked on zheap wiki page: https://wiki.postgresql.org/wiki/Zheap
You can find overall design of zheap in the README: src/backend/access/zheap/README