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Allow negative aggtransspace to indicate unbounded state size

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This patch reuses the existing aggtransspace in pg_aggregate to
signal that an aggregate's transition state can grow unboundedly.  If
aggtransspace is set to a negative value, it now indicates that the
transition state may consume unpredictable or large amounts of memory,
such as in aggregates like array_agg or string_agg that accumulate
input rows.

This information can be used by the planner to avoid applying
memory-sensitive optimizations (e.g., eager aggregation) when there is
a risk of excessive memory usage during partial aggregation.

Bump catalog version.

Per idea from Robert Haas, though applied differently than originally
suggested.

Discussion: https://postgr.es/m/CA+TgmoYbkvYwLa+1vOP7RDY7kO2=A7rppoPusoRXe44VDOGBPg@mail.gmail.com
This commit is contained in:
Richard Guo
2025-10-08 17:01:48 +09:00
parent 138da727a1
commit 185e304263
6 changed files with 21 additions and 11 deletions
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5
doc/src/sgml/catalogs.sgml
View File

@@ -596,7 +596,10 @@
</para>
<para>
Approximate average size (in bytes) of the transition state
data, or zero to use a default estimate
data. A positive value provides an estimate; zero means to
use a default estimate. A negative value indicates the state
data can grow unboundedly in size, such as when the aggregate
accumulates input rows (e.g., array_agg, string_agg).
</para></entry>
</row>

11
doc/src/sgml/ref/create_aggregate.sgml
View File

@@ -384,9 +384,13 @@ SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
<para>
The approximate average size (in bytes) of the aggregate's state value.
If this parameter is omitted or is zero, a default estimate is used
based on the <replaceable>state_data_type</replaceable>.
based on the <replaceable>state_data_type</replaceable>. If set to a
negative value, it indicates the state data can grow unboundedly in
size, such as when the aggregate accumulates input rows (e.g.,
array_agg, string_agg).
The planner uses this value to estimate the memory required for a
grouped aggregate query.
grouped aggregate query and to avoid optimizations that may cause
excessive memory usage.
</para>
</listitem>
</varlistentry>
@@ -568,7 +572,8 @@ SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
<para>
The approximate average size (in bytes) of the aggregate's state
value, when using moving-aggregate mode. This works the same as
<replaceable>state_data_size</replaceable>.
<replaceable>state_data_size</replaceable>, except that negative
values are not used to indicate unbounded state size.
</para>
</listitem>
</varlistentry>