ANALYZE: Interpreting rows and filtered members

This article describes how to interpret r_rows and r_filtered members in ANALYZE FORMAT=JSON when an index-based access method is used.

Index-based access method

Index-based access method may employ some or all of the following:

• Index Condition Pushdown
• Rowid Filtering
• attached_condition checking

Consider a table access which does all three:

  "table": {
    "table_name": "t1",
    "access_type": "range",
    "possible_keys": ...,
    "key": "INDEX1",
    ...
    "rowid_filter": {
      ...
      "r_selectivity_pct": n.nnn,
    },
    ...
    "rows": 123,
    "r_rows": 125,
    ...
    "filtered": 8.476269722,
    "r_filtered": 100,
    "index_condition": "cond1",
    "attached_condition": "cond2"
  }

The access is performed as follows:

Access diagram

Statistics values in MariaDB before 11.5

In MariaDB versions before 11.5, the counters were counted as follows:

that is,

• r_rows is counted after Index Condition Pushdown check and Rowid Filter check.
• r_filtered only counts selectivity of the attached_condition.
• selectivity of the Rowid Filter is in rowid_filter.r_selectivity_pct.

Statistics values in MariaDB 11.5 and later versions

Starting from MariaDB 11.5 (MDEV-18478), the row counters are:

• r_index_rows counts the number of enumerated index tuples, before any checks are made
• r_rows is the same as before - number of rows after index checks.

The selectivity counters are:

• r_icp_filtered is the percentage of records left after pushed index condition check.
• rowid_filter.r_selectivity_pct shows selectivity of Rowid Filter, as before.
• r_filtered is the selectivity of attached_condition check, as before.
• r_total_filtered is the combined selectivity of all checks.

in ANALYZE FORMAT=JSON output these members are placed as follows:

  "table": {
    "table_name": ...,

    "rows": 426,
    "r_index_rows": 349,
    "r_rows": 34,

Whenever applicable, r_index_rows is shown. It is comparable with rows - both are numbers of rows to enumerate before any filtering is done. If r_index_rows is not shown, r_rows shows the number of records enumerated.

Then, filtering members:

    ...
    "filtered": 8.476269722,
    "r_total_filtered": 9.742120344,

filtered is comparable with r_total_filtered: both show total amount of filtering.

    ...
    "index_condition": "lineitem.l_quantity > 47",
    "r_icp_filtered": 100,

ICP and its observed filtering. The optimizer doesn't compute an estimate for this currently.

    ...
    "attached_condition": "lineitem.l_shipDATE between '1997-01-01' and '1997-06-30'",
    "r_filtered": 100

attached_condition and its observed filtering.