Add memory-efficient stack for consolidate tasks.

Using this for MakeCcdVisitTableTask is trickier because we can't
ask an ExposureCatalog how many rows it has before loading it.  If
that's needed, we can extend this code to do it on another branch.

python/lsst/pipe/tasks/postprocess.py

@@ -44,6 +44,7 @@
 import numpy as np
 import pandas as pd
 import astropy.table
+import astropy.utils.metadata
 
 import lsst.geom
 import lsst.pex.config as pexConfig
@@ -82,6 +83,95 @@ def flattenFilters(df, noDupCols=["coord_ra", "coord_dec"], camelCase=False, inp
     return newDf
 
 
+class TableVStack:
+    """A helper class for stacking astropy tables without having them all in
+    memory at once.
+
+    Parameters
+    ----------
+    capacity : `int`
+        Full size of the final table.
+
+    Notes
+    -----
+    Unlike `astropy.table.vstack`, this class requires all tables to have the
+    exact same columns (it's slightly more strict than even the
+    ``join_type="exact"`` argument to `astropy.table.vstack`).
+    """
+
+    def __init__(self, capacity):
+        self.index = 0
+        self.capacity = capacity
+        self.result = None
+
+    @classmethod
+    def from_handles(cls, handles):
+        """Construct from an iterable of
+        `lsst.daf.butler.DeferredDatasetHandle`.
+
+        Parameters
+        ----------
+        handles : `~collections.abc.Iterable` [ \
+                `lsst.daf.butler.DeferredDatasetHandle` ]
+            Iterable of handles.   Must have a storage class that supports the
+            "rowcount" component, which is all that will be fetched.
+
+        Returns
+        -------
+        vstack : `TableVStack`
+            An instance of this class, initialized with capacity equal to the
+            sum of the rowcounts of all the given table handles.
+        """
+        capacity = sum(handle.get("rowcount") for handle in handles)
+        return cls(capacity=capacity)
+
+    def extend(self, table):
+        """Add a single table to the stack.
+
+        Parameters
+        ----------
+        table : `astropy.table.Table`
+            An astropy table instance.
+        """
+        if self.result is None:
+            self.result = astropy.table.Table()
+            for name in table.colnames:
+                column = table[name]
+                column_cls = type(column)
+                self.result[name] = column_cls.info.new_like([column], self.capacity, name=name)
+            self.index = len(table)
+            self.result.meta = table.meta.copy()
+        else:
+            next_index = self.index + len(table)
+            for name in table.colnames:
+                self.result[name][self.index:next_index] = table[name]
+            self.index = next_index
+            self.result.meta = astropy.utils.metadata.merge(self.result.meta, table.meta)
+
+    @classmethod
+    def vstack_handles(cls, handles):
+        """Vertically stack tables represented by deferred dataset handles.
+
+        Parameters
+        ----------
+        handles : `~collections.abc.Iterable` [ \
+                `lsst.daf.butler.DeferredDatasetHandle` ]
+            Iterable of handles.   Must have the "ArrowAstropy" storage class
+            and identical columns.
+
+        Returns
+        -------
+        table : `astropy.table.Table`
+            Concatenated table with the same columns as each input table and
+            the rows of all of them.
+        """
+        handles = tuple(handles)  # guard against single-pass iterators
+        vstack = cls.from_handles(handles)
+        for handle in handles:
+            vstack.extend(handle.get())
+        return vstack.result
+
+
 class WriteObjectTableConnections(pipeBase.PipelineTaskConnections,
                                   defaultTemplates={"coaddName": "deep"},
                                   dimensions=("tract", "patch", "skymap")):
@@ -932,6 +1022,7 @@ class ConsolidateObjectTableConnections(pipeBase.PipelineTaskConnections,
         storageClass="ArrowAstropy",
         dimensions=("tract", "patch", "skymap"),
         multiple=True,
+        deferLoad=True,
     )
     outputCatalog = connectionTypes.Output(
         doc="Pre-tract horizontal concatenation of the input objectTables",
@@ -965,7 +1056,7 @@ def runQuantum(self, butlerQC, inputRefs, outputRefs):
         inputs = butlerQC.get(inputRefs)
         self.log.info("Concatenating %s per-patch Object Tables",
                       len(inputs["inputCatalogs"]))
-        table = astropy.table.vstack(inputs["inputCatalogs"], join_type="exact")
+        table = TableVStack.vstack_handles(inputs["inputCatalogs"])
         butlerQC.put(pipeBase.Struct(outputCatalog=table), outputRefs)
 
 
@@ -1142,7 +1233,8 @@ class ConsolidateSourceTableConnections(pipeBase.PipelineTaskConnections,
         name="{catalogType}sourceTable",
         storageClass="ArrowAstropy",
         dimensions=("instrument", "visit", "detector"),
-        multiple=True
+        multiple=True,
+        deferLoad=True,
     )
     outputCatalog = connectionTypes.Output(
         doc="Per-visit concatenation of Source Table",
@@ -1175,7 +1267,7 @@ def runQuantum(self, butlerQC, inputRefs, outputRefs):
         inputs = butlerQC.get(inputRefs)
         self.log.info("Concatenating %s per-detector Source Tables",
                       len(inputs["inputCatalogs"]))
-        table = astropy.table.vstack(inputs["inputCatalogs"], join_type="exact")
+        table = TableVStack.vstack_handles(inputs["inputCatalogs"])
         butlerQC.put(pipeBase.Struct(outputCatalog=table), outputRefs)