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pgsql-bloat-estimation

Queries to mesure statistical bloat in btree indexes and tables for PostgreSQL.

Three different kind of non used space should be considered:

  • the alignment padding: depending on the type, PostgreSQL adds some padding to your fields to align them correctly in the row. This is related to some CPU manipulation optimisation.
  • the fillfactor: this allows you to set up a ratio of free space to keep in your tables or indexes. See the PostgreSQL documentation for more information
  • the bloat itself: this is the extra space not needed by the table or the index to keep your rows. This should be mapped and under control by autovacuum and/or your vacuum maintenance procedure.

Tables

The queries from the "table" folder estimate the bloat for tables. They expose these fields:

  • current_database: name of the current database.
  • schemaname: schema of the table.
  • tblname: the table name.
  • real_size: real size of the table.
  • extra_size: estimated extra size not used/needed in the table. This extra size is composed by the fillfactor, bloat and alignment padding spaces.
  • extra_ratio: estimated ratio of the real size used by extra_size.
  • fillfactor: the fillfactor of the table.
  • bloat_size: estimated size of the bloat without the extra space kept for the fillfactor.
  • bloat_ratio: estimated ratio of the real size used by bloat_size.
  • is_na: is the estimation "Not Applicable" ? If true, do not trust the stats.

AS 7.4, 8.0 and 8.1 do not have fillfactor, extra_size, extra_ratio and bloat_size are not reported.

BTree indexes

The queries from the "btree" folder estimate the bloat for btree indexes. They expose these fields:

  • current_database: name of the current database
  • schemaname: schema of the table
  • tblname: the table name
  • idxname: the index name
  • real_size: real size of the index
  • extra_size: estimated extra size not used/needed by the index. This extra size is composed by the fillfactor, bloat and alignment padding spaces.
  • extra_ratio: estimated ratio of the real size used by extra_size.
  • fillfactor: the fillfactor of the index.
  • bloat_size: estimated size of the bloat without the extra space kept
    for the fillfactor.
  • bloat_ratio: estimated ratio of the real size used by bloat_size.
  • is_na: is the estimation "Not Applicable" ? If true, do not trust the stats.

The is_na column

This field allows you to filter out statistics considered wrong by the query itself. Just uncomment the WHERE clause.

This actually exclude any table or index using the name type. Statistics for this type are not correlated to its space use, leading to wrong statistics.

A lot of relations from pg_catalog reports negative stats because of it.

Alignment padding

Unfortunately, as it is not possible to compute the space wasted by the alignment paddings, it is always reported in the bloat fields. Sometime, this space can takes up to 10% or more of the table size. See the chapter "The alignment deviation" from this page for more information.

This means you can estimate this space by running the query on non-bloated table. The bloat fields will then only report this alignment padding space. For large table, you can sample it in a smaller table of 100 pages or so, keeping the same field order. The bloat estimation query will report the same average space wasted by alignment padding from this table.

Size of tables/indexes

Small table or indexes (few pages) will certainly reports high bloat ratio. Each pages beeing 8kB, the less you have rows to fill them, the smaller they are, the more you will have natural spaces in there.

As example, if you need 100 rows to fill one page and your table have 150 rows, your table will be on 2 pages, 16kB. The second page having only 50 rows, You'll have a natural bloat of 4kB, 25% of your table.