Allow negative aggtransspace to indicate unbounded state size

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

2 files changed, 12 insertions, 4 deletions

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index e9095bedf21..3acc2222a87 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -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>
 
diff --git a/doc/src/sgml/ref/create_aggregate.sgml b/doc/src/sgml/ref/create_aggregate.sgml
index 222e0aa5c9d..0472ac2e874 100644
--- a/doc/src/sgml/ref/create_aggregate.sgml
+++ b/doc/src/sgml/ref/create_aggregate.sgml
@@ -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>