MySQL Query Plan

Query scans 2 tables and joins via block nested-loop Learn →; returns 3,000 rows in 186 ms. Main finding: join predicate wraps the column in CONVERT() — no index can be used.

Total time: 185.70 ms
Rows returned: 3.00K
Rows examined: 4.00K
Operators: 3

Fix first

Primary recommendation

Rewrite the join condition to compare the bare column on both sides (e.g. 'a.id = b.other_id' instead of 'CONCAT(a.id)=CONCAT(b.other_id)').

Why does this help?

wrapping a column in a function means the optimizer cannot use any index on that column — every row-pair is evaluated in the server layer, making the cost O(outer × inner). Dropping the function lets MySQL/MariaDB pick an index lookup or hash join Learn → with real selectivity, typically 100–1000× faster on mid-sized tables.

Execution plan (flamegraph)

Click a plan operator to inspect details.

Warnings (3)

ERROR
Non-sargable join predicate: CONVERT(...) in t1.d = convert(concat('r',t2.y) using utf8mb4) — a function wrapped around the join column prevents index use.

Labeled: Table scan [t1]
WARN
Full table scan: t2 (1000 rows), t1 (3000 rows)

Labeled: Table scan [t2] Table scan [t1]
WARN
Block Nested-Loop Learn → (BNL Learn →) join buffer detected — uses join_buffer_size (Extra: 'Using join buffer (Block Nested Loop)Learn →', type ALL/index/range)

Labeled: Table scan [t1]

Suggestions (3)

HIGH
Rewrite the join condition to compare the bare column on both sides (e.g. 'a.id = b.other_id' instead of 'CONCAT(a.id)=CONCAT(b.other_id)').

Why?

wrapping a column in a function means the optimizer cannot use any index on that column — every row-pair is evaluated in the server layer, making the cost O(outer × inner). Dropping the function lets MySQL/MariaDB pick an index lookup or hash join Learn → with real selectivity, typically 100–1000× faster on mid-sized tables.
HIGH
Add indexes on filter/join columns to avoid full table scans
MEDIUM
Add indexes to eliminate BNL Learn → full/index scans, or increase join_buffer_size. In MySQL 8.0.20+, set block_nested_loop Learn →=off to force hash join Learn → instead

Glossary (7 terms)

block nested loop: MySQL has no index to look up matching rows, so it reads a chunk of the first table into memory and scans the second table once per chunk. This is slow on big tables. The real fix is almost always to add an index on the join column — that converts the join into a direct lookup and the chunking goes away.; BNL: signalled by ``Using join buffer (Block Nested Loop)`` in the Extra column. MySQL fills ``join_buffer_size`` with outer rows, then scans the inner table (type=ALL/index/range) once for the whole batch. Cost grows with the outer size: O(outer/batch × inner). On MySQL 8.0.20+ the executor **rewrites this BNL to a hash join at runtime** (sql/sql_executor.cc), so the label is slightly misleading — you are usually already running hash. The ``hash_join`` optimizer_switch is defined but never checked by the planner and is effectively a no-op; setting ``block_nested_loop=off`` would kill the BNL→hash rewrite, not enable it. The real fix is an index on the join column (converts the join into eq_ref / ref) or a bigger ``join_buffer_size`` so the hash build side fits. MariaDB uses ``join_cache_hashed`` + ``join_cache_level >= 3`` to select a hashed join buffer.
full table scan: MySQL is reading every single row in this table, one by one, to check if it matches your WHERE clause. This is slow on big tables — an index on the filter column lets MySQL jump straight to the matching rows instead.; Access type ``ALL``: the storage engine returns every row of the table, applying the WHERE clause in the server layer. Cost is roughly (rows × row_evaluate_cost) + pages_read × io_block_read_cost. Signal: ``type=ALL`` in EXPLAIN, ``Handler_read_rnd_next`` climbs by the table row count. Almost always a sign of a missing index on the WHERE or join column.
hash join: MySQL builds a lookup table in memory for the smaller of the two tables, then reads the bigger one once and uses the lookup table to find matches. Much faster than the alternative (Block Nested-Loop) when neither side has a usable index.; MySQL 8.0.18+ (default in 8.0.20+). Reads the smaller input, builds an in-memory hash table keyed on the join column, then scans the larger input and probes the hash. Single pass over each side (O(m + n)). Uses ``join_buffer_size``; if the build side doesn't fit, MySQL spills to tmpdir and does a multi-pass (Grace) hash join, re-reading the probe side each pass.
index lookup: MySQL uses the index to jump straight to the rows you asked for. This is the fast path — what you want for most queries.; Access type ``ref`` or ``ref_or_null``: the optimizer uses a secondary index to fetch all rows matching an equality predicate on the indexed column. Cost is O(matching_rows). Ideal access path when the filter column is indexed.
index range scan: MySQL is reading a chunk of the index that matches your range filter (like a date range) instead of the whole thing. The narrower the range, the faster.; Access type ``range``: reads a bounded key range of a secondary index (e.g. ``WHERE created_at BETWEEN a AND b``). Fast when the range is narrow; degrades to a full scan when the range covers most of the index. With ``mrr=on`` the row-ID fetches can be reordered to turn random I/O into sequential.
join buffer size: The memory MySQL uses when it has to join tables without a handy index. If this is too small for a big join, MySQL spills to disk and slows down.; Per-join allocation, freed after the join completes. Feeds the build side of a hash join or the outer-row batches of a BNL. If the build side of a hash join doesn't fit, MySQL spills to a multi-pass (Grace) hash join. An index on the join column is almost always a bigger win than raising this.
sargable: A filter or join condition MySQL can answer using an index. Comparing a plain column to a value is sargable. Wrapping the column in a function (like CONCAT or DATE) makes it non-sargable, which means MySQL has to evaluate the function on every row — no index can help.; Short for *Search ARGument-able*: a WHERE/ON predicate is sargable if it compares the bare column to a value or another bare column (``a.id = b.user_id``, ``created_at > '2024-01-01'``). Wrapping the column in a function (``CONCAT(a.id)``, ``DATE(created_at)``, ``LOWER(email)``) breaks sargability because the optimizer would need a functional index on that exact expression. Non-sargable predicates force a per-row evaluation in the server layer and defeat every regular index.

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Raw sidecar (JSON)

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  "generated_at": "2026-04-25T12:51:22Z",
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  "plan_summary": {
    "total_time_ms": 185.701,
    "rows_sent": 3000,
    "rows_examined_estimate": 4000,
    "operator_count": 3,
    "max_depth": 2
  },
  "optimizer_switches": [
    {
      "name": "block_nested_loop",
      "value": "on",
      "explanation": "Buffers a batch of outer rows in a join buffer and scans the inner table once per batch instead of once per row. Inner still does a full/index/range scan — a missing index on the join column is the root cause.",
      "node_labels": [
        "Table scan [t1]"
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    }
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  "warnings": [
    {
      "severity": "error",
      "category": "nonsargable_join",
      "text": "Non-sargable join predicate: CONVERT(...) in t1.d = convert(concat('r',t2.y) using utf8mb4) — a function wrapped around the join column prevents index use.",
      "source": "plan",
      "node_labels": [
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      "severity": "warn",
      "category": "full_scan",
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      "source": "plan",
      "node_labels": [
        "Table scan [t2]",
        "Table scan [t1]"
      ]
    },
    {
      "severity": "warn",
      "category": "bnl",
      "text": "Block Nested-Loop (BNL) join buffer detected — uses join_buffer_size (Extra: 'Using join buffer (Block Nested Loop)', type ALL/index/range)",
      "source": "plan",
      "node_labels": [
        "Table scan [t1]"
      ]
    }
  ],
  "suggestions": [
    {
      "severity": "high",
      "category": "rewrite",
      "action": "Rewrite the join condition to compare the bare column on both sides (e.g. 'a.id = b.other_id' instead of 'CONCAT(a.id)=CONCAT(b.other_id)').",
      "source": "plan",
      "why": "wrapping a column in a function means the optimizer cannot use any index on that column — every row-pair is evaluated in the server layer, making the cost O(outer × inner). Dropping the function lets MySQL/MariaDB pick an index lookup or hash join with real selectivity, typically 100–1000× faster on mid-sized tables."
    },
    {
      "severity": "high",
      "category": "index",
      "action": "Add indexes on filter/join columns to avoid full table scans",
      "source": "plan"
    },
    {
      "severity": "medium",
      "category": "tuning_variable",
      "action": "Add indexes to eliminate BNL full/index scans, or increase join_buffer_size. In MySQL 8.0.20+, set block_nested_loop=off to force hash join instead",
      "source": "plan",
      "target_variable": "join_buffer_size"
    }
  ],
  "executive_summary": "Query scans 2 tables and joins via block nested-loop; returns 3,000 rows in 186 ms. Main finding: join predicate wraps the column in CONVERT() — no index can be used.",
  "plan_tree": {
    "node_id": "n:4ee53c8ca1ad",
    "short_label": "Nested loop inner join",
    "folded_label": "NESTED LOOP INNER starts=1 rows=3000",
    "children": [
      {
        "node_id": "n:7561bc6d0312",
        "short_label": "Table scan [t2]",
        "folded_label": "TABLE SCAN [t2] starts=1 rows=1000",
        "children": []
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        "node_id": "n:d36789d76d9a",
        "short_label": "Table scan [t1]",
        "folded_label": "TABLE SCAN [t1] starts=1 rows=3000",
        "children": []
      }
    ]
  },
  "index_suggestions": [
    {
      "table": "t1",
      "columns": [
        "d"
      ],
      "ddl": "CREATE INDEX idx_t1_d ON t1 (d);",
      "reason": "Full scan on t1 with filter on (d)"
    }
  ],
  "primary_action": {
    "ref": "suggestions[0]"
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      "lesson": "nested_loop",
      "match": {
        "folded_label": "NESTED LOOP INNER starts=1 rows=3000",
        "short_label": "Nested loop inner join"
      },
      "controls": {
        "outer_rows": 1000,
        "inner_rows": 3000,
        "row_size": 200
      },
      "note": "Nested loop inner join"
    },
    {
      "lesson": "full_scan",
      "match": {
        "folded_label": "TABLE SCAN [t2] starts=1 rows=1000",
        "short_label": "Table scan [t2]"
      },
      "controls": {
        "rows": 1000,
        "row_size": 200,
        "selectivity": 100.0
      },
      "note": "Table scan on t2 (t2)"
    },
    {
      "lesson": "bnl",
      "match": {
        "folded_label": "TABLE SCAN [t1] starts=1 rows=3000",
        "short_label": "Table scan [t1]"
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      "controls": {
        "outer_rows": 3000,
        "inner_rows": 3000,
        "row_size": 200
      },
      "note": "Table scan on t1 (t1)"
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      "node_id": "n:4ee53c8ca1ad",
      "folded_label": "NESTED LOOP INNER starts=1 rows=3000",
      "short_label": "Nested loop inner join",
      "complexity": {
        "big_o": "O(n · m)",
        "short": "n · m",
        "severity": "bad",
        "rationale": "Unindexed nested loop: every outer row drives a full scan of the inner side. This is the classic O(n²)-class blow-up.",
        "confidence": "exact",
        "learn_more": "nested_loop_join"
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      "node_id": "n:7561bc6d0312",
      "folded_label": "TABLE SCAN [t2] starts=1 rows=1000",
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      "node_id": "n:d36789d76d9a",
      "folded_label": "TABLE SCAN [t1] starts=1 rows=3000",
      "short_label": "Table scan [t1]",
      "complexity": {
        "big_o": "O(n)",
        "short": "n",
        "severity": "medium",
        "rationale": "Full table scan: the storage engine returns every row; cost scales with the table size.",
        "confidence": "exact",
        "learn_more": "full_table_scan"
      }
    }
  ]
}

Primary recommendation

Execution plan (flamegraph)

Warnings (3)

Suggestions (3)

myteach — interactive algorithm catalog

Lessons relevant to this plan

Raw sidecar (JSON)

Nested Loop Join — outer row drives inner probeMySQL 8.4 • MariaDB 11.4

Join shape controls

Nested loop cost model

Full table scan — why MySQL reads every rowMySQL 8.4 • MariaDB 11.4

Parameters (full table scan cost model)

Cost readout (full table scan)

Block Nested Loop join — MariaDB's defaultMariaDB 11.4

Parameters (MariaDB 11.x Block Nested Loop)

Cost readout (MariaDB 11.x BNL)

Teach: Operator deep dive

Summary

Execution plan (flamegraph)

Warnings (3)

Suggestions (3)

myteach — interactive algorithm catalog

Lessons relevant to this plan

Raw sidecar (JSON)