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Description

This error occurs when you attempt to create a functional index on an expression or function that returns a JSON or GEOMETRY data type. MySQL explicitly prohibits functional indexes on these types due to their complex structure and the overhead involved in evaluating them during indexing operations.

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Error Message

Cannot create a functional index on a function that returns a JSON or GEOMETRY value.

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Known Causes

3 known causes

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Indexing JSON Data

You attempted to create a functional index on an expression that extracts or manipulates JSON data from a column, which is not supported.

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Indexing GEOMETRY Data

A functional index was defined on an expression involving GEOMETRY data types, which are explicitly disallowed for this indexing method.

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Unsupported Function Return Type

The function used in the functional index definition, although valid for other operations, returns a JSON or GEOMETRY value, making it incompatible.

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Solutions

4 solutions available

1. Extract JSON Values to Separate Columns medium

Denormalize JSON data into dedicated columns for indexing.

1

Identify the specific JSON keys you need to query frequently and create new columns in your table to store these extracted values.

ALTER TABLE your_table ADD COLUMN extracted_key VARCHAR(255);

2

Populate these new columns by extracting the relevant data from your JSON column.

UPDATE your_table SET extracted_key = JSON_EXTRACT(json_column, '$.your_key');

3

Create a regular index on the newly created columns.

CREATE INDEX idx_extracted_key ON your_table (extracted_key);

4

Modify your queries to use the new indexed columns instead of the JSON functions.

SELECT * FROM your_table WHERE extracted_key = 'some_value';

2. Use Generated Columns for Indexing medium

Leverage generated columns that store the result of a JSON extraction, which can then be indexed.

1

Add a generated column to your table. This column will automatically store the extracted value from your JSON.

ALTER TABLE your_table ADD COLUMN extracted_key_generated INT AS (JSON_EXTRACT(json_column, '$.your_key')) STORED;

2

Create an index on the generated column.

CREATE INDEX idx_extracted_key_generated ON your_table (extracted_key_generated);

3

Update your queries to use the generated column.

SELECT * FROM your_table WHERE extracted_key_generated = 123;

3. Pre-calculate and Store Derived Data advanced

For complex JSON or GEOMETRY manipulations, calculate and store the result in a separate, indexable column.

1

Determine the specific derived values you need to query frequently from your JSON or GEOMETRY data.

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Example: If you frequently query the area of a GEOMETRY polygon, you would calculate this area.

2

Add a new column to your table to store these pre-calculated values.

ALTER TABLE your_table ADD COLUMN derived_value DECIMAL(10, 2);

3

Use triggers or application logic to populate and maintain this new column whenever the original JSON or GEOMETRY data is inserted or updated.

CREATE TRIGGER update_derived_value BEFORE INSERT ON your_table
FOR EACH ROW SET NEW.derived_value = ST_Area(NEW.geometry_column);

4

Create an index on the new `derived_value` column.

CREATE INDEX idx_derived_value ON your_table (derived_value);

5

Modify your queries to filter on the `derived_value` column.

SELECT * FROM your_table WHERE derived_value > 1000;

4. Re-evaluate Indexing Strategy easy

Consider if indexing the specific JSON/GEOMETRY function is truly necessary or if alternative query patterns exist.

1

Analyze the queries that are failing. Understand what specific information is being sought from the JSON or GEOMETRY data.

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Example: Are you searching for a specific value within a JSON object, or checking if a point falls within a GEOMETRY polygon?

2

Explore if simpler, non-functional indexable queries can achieve the same result. This might involve scanning the JSON/GEOMETRY data directly if the volume is manageable.

SELECT * FROM your_table WHERE JSON_EXTRACT(json_column, '$.some_key') = 'value'; -- This query would fail if used in a functional index.

3

If performance is critical and direct indexing isn't possible, consider the other solutions provided to denormalize or pre-calculate data.

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No code snippet for this step as it's a strategic decision.

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Related Errors

5 related errors

MySQL Error 3753: Functional Index on JSON/GEOMETRY