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doc: explain use of json_populate_record{set}()

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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
This commit is contained in:
Bruce Momjian 2018-06-19 13:43:40 -04:00
parent 1bebfb9b63
commit 4832c4c191
1 changed files with 21 additions and 7 deletions
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28
doc/src/sgml/func.sgml
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@ -11457,14 +11457,28 @@ table2-mapping
<note>
<para>
In <function>json_populate_record</>, <function>json_populate_recordset</>,
<function>json_to_record</> and <function>json_to_recordset</>,
type coercion from the JSON is <quote>best effort</> and may not result
in desired values for some types. JSON keys are matched to
identical column names in the target row type. JSON fields that do not
appear in the target row type will be omitted from the output, and
target columns that do not match any JSON field will simply be NULL.
While the examples for the functions
<function>json_populate_record</function>,
<function>json_populate_recordset</function>,
<function>json_to_record</function> and
<function>json_to_recordset</function> use constants, the typical use
would be to reference a table in the <literal>FROM</literal> clause
and use one of its <type>json</type> or <type>jsonb</type> columns
as an argument to the function. Extracted key values can then be
referenced in other parts of the query, like <literal>WHERE</literal>
clauses and target lists. Extracting multiple values in this
way can improve performance over extracting them separately with
per-key operators.
</para>
<para>
JSON keys are matched to identical column names in the target
row type. JSON type coercion for these functions is <quote>best
effort</quote> and may not result in desired values for some types.
JSON fields that do not appear in the target row type will be
omitted from the output, and target columns that do not match any
JSON field will simply be NULL.
</para>
</note>
<note>