114 lines
5.1 KiB
Plaintext
114 lines
5.1 KiB
Plaintext
[role="xpack"]
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[testenv="basic"]
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[[sql-limitations]]
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== SQL Limitations
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[float]
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=== Nested fields in `SYS COLUMNS` and `DESCRIBE TABLE`
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{es} has a special type of relationship fields called `nested` fields. In {es-sql} they can be used by referencing their inner
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sub-fields. Even though `SYS COLUMNS` and `DESCRIBE TABLE` will still display them as having the type `NESTED`, they cannot
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be used in a query. One can only reference its sub-fields in the form:
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[source, sql]
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--------------------------------------------------
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[nested_field_name].[sub_field_name]
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--------------------------------------------------
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For example:
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[source, sql]
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--------------------------------------------------
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SELECT dep.dep_name.keyword FROM test_emp GROUP BY languages;
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--------------------------------------------------
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[float]
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=== Multi-nested fields
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{es-sql} doesn't support multi-nested documents, so a query cannot reference more than one nested field in an index.
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This applies to multi-level nested fields, but also multiple nested fields defined on the same level. For example, for this index:
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[source, sql]
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----------------------------------------------------
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column | type | mapping
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----------------------+---------------+-------------
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nested_A |STRUCT |NESTED
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nested_A.nested_X |STRUCT |NESTED
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nested_A.nested_X.text|VARCHAR |KEYWORD
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nested_A.text |VARCHAR |KEYWORD
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nested_B |STRUCT |NESTED
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nested_B.text |VARCHAR |KEYWORD
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----------------------------------------------------
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`nested_A` and `nested_B` cannot be used at the same time, nor `nested_A`/`nested_B` and `nested_A.nested_X` combination.
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For such situations, {es-sql} will display an error message.
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[float]
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=== Paginating nested inner hits
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When SELECTing a nested field, pagination will not work as expected, {es-sql} will return __at least__ the page size records.
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This is because of the way nested queries work in {es}: the root nested field will be returned and it's matching inner nested fields as well,
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pagination taking place on the **root nested document and not on its inner hits**.
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[float]
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=== Normalized `keyword` fields
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`keyword` fields in {es} can be normalized by defining a `normalizer`. Such fields are not supported in {es-sql}.
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[float]
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=== Array type of fields
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Array fields are not supported due to the "invisible" way in which {es} handles an array of values: the mapping doesn't indicate whether
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a field is an array (has multiple values) or not, so without reading all the data, {es-sql} cannot know whether a field is a single or multi value.
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[float]
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=== Sorting by aggregation
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When doing aggregations (`GROUP BY`) {es-sql} relies on {es}'s `composite` aggregation for its support for paginating results.
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However this type of aggregation does come with a limitation: sorting can only be applied on the key used for the aggregation's buckets.
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{es-sql} overcomes this limitation by doing client-side sorting however as a safety measure, allows only up to *512* rows.
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It is recommended to use `LIMIT` for queries that use sorting by aggregation, essentially indicating the top N results that are desired:
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[source, sql]
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--------------------------------------------------
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SELECT * FROM test GROUP BY age ORDER BY COUNT(*) LIMIT 100;
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--------------------------------------------------
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It is possible to run the same queries without a `LIMIT` however in that case if the maximum size (*512*) is passed, an exception will be
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returned as {es-sql} is unable to track (and sort) all the results returned.
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[float]
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=== Using aggregation functions on top of scalar functions
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Aggregation functions like <<sql-functions-aggs-min,`MIN`>>, <<sql-functions-aggs-max,`MAX`>>, etc. can only be used
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directly on fields, and so queries like `SELECT MAX(abs(age)) FROM test` are not possible.
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[float]
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=== Using a sub-select
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Using sub-selects (`SELECT X FROM (SELECT Y)`) is **supported to a small degree**: any sub-select that can be "flattened" into a single
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`SELECT` is possible with {es-sql}. For example:
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["source","sql",subs="attributes,macros"]
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--------------------------------------------------
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include-tagged::{sql-specs}/docs.csv-spec[limitationSubSelect]
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--------------------------------------------------
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The query above is possible because it is equivalent with:
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["source","sql",subs="attributes,macros"]
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--------------------------------------------------
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include-tagged::{sql-specs}/docs.csv-spec[limitationSubSelectRewritten]
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--------------------------------------------------
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But, if the sub-select would include a `GROUP BY` or `HAVING` or the enclosing `SELECT` would be more complex than `SELECT X
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FROM (SELECT ...) WHERE [simple_condition]`, this is currently **un-supported**.
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[float]
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=== Using <<sql-functions-aggs-first, `FIRST`>>/<<sql-functions-aggs-last,`LAST`>> aggregation functions in `HAVING` clause
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Using `FIRST` and `LAST` in the `HAVING` clause is not supported. The same applies to
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<<sql-functions-aggs-min,`MIN`>> and <<sql-functions-aggs-max,`MAX`>> when their target column
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is of type <<keyword, `keyword`>> as they are internally translated to `FIRST` and `LAST`.
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