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ANALYZE: Interpreting execution stats for index-based access methods

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This article describes how to interpret r_rows and r_filtered members in ANALYZE FORMAT=JSON output for cases when the table uses index-based access method.

Index-based access method may employ

  • Index Condition Pushdown
  • Rowid Filtering
  • Regular attached_condition checking

Consider ANALYZE FORMAT=JSON output listing all of the above:

  "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"
  }

Access diagram

The access is performed as follows:

index-read-diagram-3

Statistics before the fix for MDEV-18478

Before the fix for MDEV-18478, the counters were counted as follows:

PIC2

that is,

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

Statistics after the fix for MDEV-18478

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