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doc: explain use of json_populate_record{set}()
The set-returning nature of these functions make their use unclear. The modified paragraph was added in PG 9.4. Reported-by: yshaladi@denodo.com Discussion: https://postgr.es/m/152571684246.9460.18059951267371255159@wrigleys.postgresql.org Backpatch-through: 9.4
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@ -11457,14 +11457,28 @@ table2-mapping
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<note>
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<para>
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In <function>json_populate_record</>, <function>json_populate_recordset</>,
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<function>json_to_record</> and <function>json_to_recordset</>,
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type coercion from the JSON is <quote>best effort</> and may not result
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in desired values for some types. JSON keys are matched to
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identical column names in the target row type. JSON fields that do not
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appear in the target row type will be omitted from the output, and
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target columns that do not match any JSON field will simply be NULL.
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While the examples for the functions
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<function>json_populate_record</function>,
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<function>json_populate_recordset</function>,
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<function>json_to_record</function> and
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<function>json_to_recordset</function> use constants, the typical use
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would be to reference a table in the <literal>FROM</literal> clause
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and use one of its <type>json</type> or <type>jsonb</type> columns
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as an argument to the function. Extracted key values can then be
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referenced in other parts of the query, like <literal>WHERE</literal>
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clauses and target lists. Extracting multiple values in this
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way can improve performance over extracting them separately with
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per-key operators.
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</para>
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<para>
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JSON keys are matched to identical column names in the target
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row type. JSON type coercion for these functions is <quote>best
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effort</quote> and may not result in desired values for some types.
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JSON fields that do not appear in the target row type will be
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omitted from the output, and target columns that do not match any
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JSON field will simply be NULL.
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</para>
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</note>
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<note>
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