Tag «Planet PostgreSQL»


MVCC in PostgreSQL — 6. Vacuum

We started with problems related to isolation, made a digression about low-level data structure, then discussed row versions and observed how data snapshots are obtained from row versions.

Last time we talked about HOT updates and in-page vacuuming, and today we'll proceed to a well-known vacuum vulgaris. Really, so much has already been written about it that I can hardly add anything new, but the beauty of a full picture requires sacrifice. So keep patience.


What does vacuum do?

In-page vacuum works fast, but frees only part of the space. It works within one table page and does not touch indexes.

The basic, "normal" vacuum is done using the VACUUM command, and we will call it just "vacuum" (leaving "autovacuum" for a separate discussion).

So, vacuum processes the entire table. It vacuums away not only dead tuples, but also references to them from all indexes.

Vacuuming is concurrent with other activities in the system. The table and indexes can be used in a regular way both for reads and updates (however, concurrent execution of commands such as CREATE INDEX, ALTER TABLE and some others is impossible).

Only those table pages are looked through where some activities took place. To detect them, the visibility map is used (to remind you, the map tracks those pages that contain pretty old tuples, which are visible in all data snapshots for sure). Only those pages are processed that are not tracked by the visibility map, and the map itself gets updated.

The free space map also gets updated in the process to reflect the extra free space in the pages.


MVCC in PostgreSQL — 5. In-page vacuum and HOT updates

Just to remind you, we already discussed issues related to isolation, made a digression regarding low-level data structure, and then explored row versions and observed how data snapshots are obtained from row versions.

Now we will proceed to two closely connected problems: in-page vacuum и HOT updates. Both techniques can be referred to optimizations; they are important, but virtually not covered in the documentation.

In-page vacuum during regular updates

When accessing a page for either an update or read, if PostgreSQL understands that the page is running out of space, it can do a fast in-page vacuum. This happens in either of the cases:

  1. A previous update in this page did not find enough space to allocate a new row version in the same page. Such a situation is remembered in the page header, and next time the page is vacuumed.
  2. The page is more than fillfactor percent full. In this case, vacuum is performed right away without putting off till next time.

MVCC in PostgreSQL — 4. Snapshots

After having discussed isolation problems and having made a digression regarding the low-level data structure, last time we explored row versions and observed how different operations changed tuple header fields.

Now we will look at how consistent data snapshots are obtained from tuples.

What is a data snapshot?

Data pages can physically contain several versions of the same row. But each transaction must see only one (or none) version of each row, so that all of them make up a consistent picture of the data (in the sense of ACID) as of a certain point in time.

Isolation in PosgreSQL is based on snapshots: each transaction works with its own data snapshot, which "contains" data that were committed before the moment the snapshot was created and does not "contain" data that were not committed by that moment yet. We've already seen that although the resulting isolation appears stricter than required by the standard, it still has anomalies.

MVCC in PostgreSQL — 3. Row Versions

Well, we've already discussed isolation and made a digression regarding the low-level data structure. And we've finally reached the most fascinating thing, that is, row versions (tuples).

Tuple header

As already mentioned, several versions of each row can be simultaneously available in the database. And we need to somehow distinguish one version from another one. To this end, each version is labeled with its effective "time" (xmin) and expiration "time" (xmax). Quotation marks denote that a special incrementing counter is used rather than the time itself. And this counter is the transaction identifier.

(As usual, in reality this is more complicated: the transaction ID cannot always increment due to a limited bit depth of the counter. But we will explore more details of this when our discussion reaches freezing.)

When a row is created, the value of xmin is set equal to the ID of the transaction that performed the INSERT command, while xmax is not filled in.

When a row is deleted, the xmax value of the current version is labeled with the ID of the transaction that performed DELETE.

An UPDATE command actually performs two subsequent operations: DELETE and INSERT. In the current version of the row, xmax is set equal to the ID of the transaction that performed UPDATE. Then a new version of the same row is created, in which the value of xmin is the same as xmax of the previous version.