diff --git a/src/columnar.h b/src/columnar.h
index 84c68227..16345ec1 100644
--- a/src/columnar.h
+++ b/src/columnar.h
@@ -344,26 +344,8 @@ typedef struct PgColumnarProjection
  * ------------------------------------------------------------------------- */
 struct SMgrRelationData;
 
-extern void PgColumnarWriteNewMetapage(const RelFileLocator *newrlocator,
-									 struct SMgrRelationData *srel,
-									 char persistence, uint64 storageId);
 extern void PgColumnarReadMetapage(Relation rel, PgColumnarMetapage *meta);
 extern uint64 PgColumnarStorageId(Relation rel);
-extern void PgColumnarEnsureStorageRow(Relation rel);	/* pre-create storage row (#300 parallel_copy) */
-extern void PgColumnarReserveRowNumbers(Relation rel, uint64 rowCount,
-									  uint64 *stripeId, uint64 *firstRowNumber);
-extern void PgColumnarReserveOffset(Relation rel, uint64 dataLength,
-								  uint64 *fileOffset);
-extern void PgColumnarAdvanceReservedOffset(Relation rel, uint64 addBytes);
-extern void PgColumnarDebugSetMetapageVersion(Relation rel, uint32 versionMajor,
-											uint32 versionMinor);
-extern void PgColumnarSetReservedOffset(Relation rel, uint64 newOffset);
-extern void PgColumnarTruncateMainFork(Relation rel, BlockNumber newnblocks);
-extern void PgColumnarWriteLogicalData(Relation rel, uint64 logicalOffset,
-									 char *data, uint64 length);
-extern void PgColumnarReadLogicalData(Relation rel, uint64 logicalOffset,
-									char *dest, uint64 length);
-extern void PgColumnarResetMetapage(Relation rel);
 
 /* row number <-> item pointer (spec 6) */
 extern void PgColumnarRowNumberToItemPointer(uint64 rowNumber, ItemPointer tid);
@@ -453,8 +435,7 @@ extern void PgColumnarCheckFreeSpaceNoOverlap(uint64 storageId);
 extern uint64 PgColumnarNextStorageId(void);
 
 /* projection: needed attnos (pull_varattnos form) -> the reader's 0-based set */
-extern Bitmapset *PgColumnarProjectionFromAttnos(Bitmapset *needed, int natts,
-											   int *nProjected);
+
 extern void PgColumnarCheckNativeFormatVersion(uint64 storageId, const char *relName);
 extern List *PgColumnarReadRowGroupList(uint64 storageId, Snapshot snapshot);
 extern List *PgColumnarReadZoneMapList(uint64 storageId, uint64 groupNumber,
@@ -503,37 +484,17 @@ extern void PgColumnarUpsertDeleteVector(uint64 storageId, DeleteVectorMetadata
 typedef struct PgColumnarWriteState PgColumnarWriteState;
 
 extern PgColumnarWriteState *PgColumnarGetWriteState(Relation rel);
-extern int PgColumnarWriteStateStripeCount(PgColumnarWriteState *ws);
-extern uint64 *PgColumnarWriteStateStripeIds(PgColumnarWriteState *ws, int *n);
-extern uint64 *PgColumnarWriteStateProjStripeIds(PgColumnarWriteState *ws, int *n);
 extern uint64 PgColumnarWriteRow(PgColumnarWriteState *writeState, Relation rel,
 							   Datum *values, bool *nulls);
 extern void PgColumnarProjectionFanoutRow(Relation rel, PgColumnarWriteState *baseWs,
 										uint64 rowNumber, Datum *values,
 										bool *nulls);
-extern void PgColumnarBackfillProjection(Relation rel,
-									   const PgColumnarProjection *proj);
-extern bool PgColumnarBufferedRowByNumber(Relation rel, uint64 rowNumber,
-										Datum *values, bool *nulls);
 extern void PgColumnarFlushWriteStateForRelation(Oid relid);
-extern void PgColumnarForgetWriteStateForRelation(Oid relid);
-extern void PgColumnarFlushAllPendingWrites(void);
-extern void PgColumnarDiscardAllPendingWrites(void);
-extern void PgColumnarWriteStateDiscardSubXact(SubTransactionId subid);
-extern void PgColumnarWriteStatePromoteSubXact(SubTransactionId subid,
-											 SubTransactionId parent);
 
 /* -------------------------------------------------------------------------
  * delete vector / delete tracking (pgcolumnar_delete_vector.c, spec 7.5, 9)
  * ------------------------------------------------------------------------- */
-extern void PgColumnarMarkRowDeleted(Relation rel, uint64 rowNumber);
-extern bool PgColumnarDeleteVectorBufferedDeleted(Relation rel, uint64 rowNumber);
 extern void PgColumnarFlushDeleteVectorForRelation(Relation rel);
-extern void PgColumnarFlushAllDeleteVectors(void);
-extern void PgColumnarDiscardAllDeleteVectors(void);
-extern void PgColumnarDeleteVectorDiscardSubXact(SubTransactionId subid);
-extern void PgColumnarDeleteVectorPromoteSubXact(SubTransactionId subid,
-										  SubTransactionId parent);
 
 /* -------------------------------------------------------------------------
  * reader (pgcolumnar_reader.c)
@@ -831,7 +792,6 @@ extern void PgColumnarUniqueInit(void);
 /* -------------------------------------------------------------------------
  * planner integration (pgcolumnar_customscan.c, spec 8.3, 9)
  * ------------------------------------------------------------------------- */
-extern void PgColumnarCustomScanInit(void);
 
 /*
  * The single registered CustomScanMethods, shared by the base custom scan and
@@ -899,12 +859,6 @@ typedef struct PgColumnarGroupStats
 	uint64		vectorsRuledOutByValue;
 } PgColumnarGroupStats;
 
-extern void PgColumnarExplainPushedDown(int64 nfilters, ExplainState *es);
-extern void PgColumnarExplainVectorPredicates(int64 npreds, ExplainState *es);
-extern int	PgColumnarCountScanKeys(List *qual, Index scanrelid,
-								  TupleDesc tupdesc);
-extern void PgColumnarExplainGroupStats(const PgColumnarGroupStats *stats,
-									  ExplainState *es);
 
 extern ScanKey PgColumnarBuildScanKeys(List *qual, Index scanrelid,
 									 TupleDesc tupdesc, int *nkeys);
diff --git a/src/columnar_customscan.c b/src/columnar_customscan.c
index 61c4121f..8cff2232 100644
--- a/src/columnar_customscan.c
+++ b/src/columnar_customscan.c
@@ -26,6 +26,7 @@
  */
 #include "columnar.h"
 
+#include "columnar_customscan.h"
 #include "columnar_reader.h"
 #include <math.h>
 
diff --git a/src/columnar_customscan.h b/src/columnar_customscan.h
new file mode 100644
index 00000000..5c5d674c
--- /dev/null
+++ b/src/columnar_customscan.h
@@ -0,0 +1,37 @@
+/*-------------------------------------------------------------------------
+ *
+ * columnar_customscan.h
+ *		The planner side: what columnar_customscan.c offers the table AM and the vectorized path.
+ *
+ * Split out of columnar.h (#496). Every declaration here had exactly ONE
+ * consumer outside its defining file, so it was a private arrangement between
+ * two files that the other twenty were forced to recompile for.
+ *
+ * The shared vocabulary these signatures take stays in columnar.h, which this
+ * includes.
+ *
+ * Written fresh for pgColumnar.
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PGCOLUMNAR_CUSTOMSCAN_H
+#define PGCOLUMNAR_CUSTOMSCAN_H
+
+#include "columnar.h"
+
+extern Bitmapset *PgColumnarProjectionFromAttnos(Bitmapset *needed, int natts,
+											   int *nProjected);
+
+extern void PgColumnarCustomScanInit(void);
+
+extern void PgColumnarExplainPushedDown(int64 nfilters, ExplainState *es);
+
+extern void PgColumnarExplainVectorPredicates(int64 npreds, ExplainState *es);
+
+extern int	PgColumnarCountScanKeys(List *qual, Index scanrelid,
+								  TupleDesc tupdesc);
+
+extern void PgColumnarExplainGroupStats(const PgColumnarGroupStats *stats,
+									  ExplainState *es);
+
+#endif							/* PGCOLUMNAR_CUSTOMSCAN_H */
diff --git a/src/columnar_delete_vector.c b/src/columnar_delete_vector.c
index 792288da..8520afbd 100644
--- a/src/columnar_delete_vector.c
+++ b/src/columnar_delete_vector.c
@@ -21,6 +21,7 @@
  */
 #include "columnar.h"
 
+#include "columnar_delete_vector.h"
 #include "access/xact.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
diff --git a/src/columnar_delete_vector.h b/src/columnar_delete_vector.h
new file mode 100644
index 00000000..5168bff7
--- /dev/null
+++ b/src/columnar_delete_vector.h
@@ -0,0 +1,35 @@
+/*-------------------------------------------------------------------------
+ *
+ * columnar_delete_vector.h
+ *		Row deletion: what columnar_delete_vector.c offers the reader and the table AM.
+ *
+ * Split out of columnar.h (#496). Every declaration here had exactly ONE
+ * consumer outside its defining file, so it was a private arrangement between
+ * two files that the other twenty were forced to recompile for.
+ *
+ * The shared vocabulary these signatures take stays in columnar.h, which this
+ * includes.
+ *
+ * Written fresh for pgColumnar.
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PGCOLUMNAR_DELETE_VECTOR_H
+#define PGCOLUMNAR_DELETE_VECTOR_H
+
+#include "columnar.h"
+
+extern void PgColumnarMarkRowDeleted(Relation rel, uint64 rowNumber);
+
+extern bool PgColumnarDeleteVectorBufferedDeleted(Relation rel, uint64 rowNumber);
+
+extern void PgColumnarFlushAllDeleteVectors(void);
+
+extern void PgColumnarDiscardAllDeleteVectors(void);
+
+extern void PgColumnarDeleteVectorDiscardSubXact(SubTransactionId subid);
+
+extern void PgColumnarDeleteVectorPromoteSubXact(SubTransactionId subid,
+										  SubTransactionId parent);
+
+#endif							/* PGCOLUMNAR_DELETE_VECTOR_H */
diff --git a/src/columnar_parallel_copy.c b/src/columnar_parallel_copy.c
index e8bd6763..fc9e8134 100644
--- a/src/columnar_parallel_copy.c
+++ b/src/columnar_parallel_copy.c
@@ -43,6 +43,7 @@
 
 #include "columnar.h"
 
+#include "columnar_write_state.h"
 #include "access/relation.h"
 #include "access/table.h"
 #include "access/twophase.h"
diff --git a/src/columnar_projection.c b/src/columnar_projection.c
index 78629702..e33714a6 100644
--- a/src/columnar_projection.c
+++ b/src/columnar_projection.c
@@ -23,6 +23,7 @@
 #include "columnar.h"
 
 #include "columnar_metadata.h"
+#include "columnar_write_state.h"
 #include "access/table.h"
 #include "catalog/pg_type.h"
 #include "funcapi.h"
diff --git a/src/columnar_reader.c b/src/columnar_reader.c
index 37237cd2..3971647f 100644
--- a/src/columnar_reader.c
+++ b/src/columnar_reader.c
@@ -14,8 +14,10 @@
  */
 #include "columnar.h"
 
+#include "columnar_delete_vector.h"
 #include "columnar_metadata.h"
 #include "columnar_reader.h"
+#include "columnar_storage.h"
 #include "fmgr.h"
 #include "access/detoast.h"
 #include "access/htup_details.h"
diff --git a/src/columnar_storage.c b/src/columnar_storage.c
index 2bce800d..e930ea3c 100644
--- a/src/columnar_storage.c
+++ b/src/columnar_storage.c
@@ -12,6 +12,7 @@
  */
 #include "columnar.h"
 
+#include "columnar_storage.h"
 #include "fmgr.h"
 #include "columnar_compat.h"
 #include "access/rmgr.h"
diff --git a/src/columnar_storage.h b/src/columnar_storage.h
new file mode 100644
index 00000000..ed4cff4b
--- /dev/null
+++ b/src/columnar_storage.h
@@ -0,0 +1,49 @@
+/*-------------------------------------------------------------------------
+ *
+ * columnar_storage.h
+ *		The physical layer: what columnar_storage.c offers for metapages, offsets and logical data.
+ *
+ * Split out of columnar.h (#496). Every declaration here had exactly ONE
+ * consumer outside its defining file, so it was a private arrangement between
+ * two files that the other twenty were forced to recompile for.
+ *
+ * The shared vocabulary these signatures take stays in columnar.h, which this
+ * includes.
+ *
+ * Written fresh for pgColumnar.
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PGCOLUMNAR_STORAGE_H
+#define PGCOLUMNAR_STORAGE_H
+
+#include "columnar.h"
+
+extern void PgColumnarWriteNewMetapage(const RelFileLocator *newrlocator,
+									 struct SMgrRelationData *srel,
+									 char persistence, uint64 storageId);
+
+extern void PgColumnarReserveRowNumbers(Relation rel, uint64 rowCount,
+									  uint64 *stripeId, uint64 *firstRowNumber);
+
+extern void PgColumnarReserveOffset(Relation rel, uint64 dataLength,
+								  uint64 *fileOffset);
+
+extern void PgColumnarAdvanceReservedOffset(Relation rel, uint64 addBytes);
+
+extern void PgColumnarDebugSetMetapageVersion(Relation rel, uint32 versionMajor,
+											uint32 versionMinor);
+
+extern void PgColumnarSetReservedOffset(Relation rel, uint64 newOffset);
+
+extern void PgColumnarTruncateMainFork(Relation rel, BlockNumber newnblocks);
+
+extern void PgColumnarWriteLogicalData(Relation rel, uint64 logicalOffset,
+									 char *data, uint64 length);
+
+extern void PgColumnarReadLogicalData(Relation rel, uint64 logicalOffset,
+									char *dest, uint64 length);
+
+extern void PgColumnarResetMetapage(Relation rel);
+
+#endif							/* PGCOLUMNAR_STORAGE_H */
diff --git a/src/columnar_tableam.c b/src/columnar_tableam.c
index 15f8d6c6..7001ab6b 100644
--- a/src/columnar_tableam.c
+++ b/src/columnar_tableam.c
@@ -13,8 +13,12 @@
  */
 #include "columnar.h"
 
+#include "columnar_customscan.h"
+#include "columnar_delete_vector.h"
 #include "columnar_metadata.h"
 #include "columnar_reader.h"
+#include "columnar_storage.h"
+#include "columnar_write_state.h"
 #include "access/multixact.h"
 #include "access/genam.h"
 #include "access/table.h"
diff --git a/src/columnar_vacuum.c b/src/columnar_vacuum.c
index 118711a8..d87d38f3 100644
--- a/src/columnar_vacuum.c
+++ b/src/columnar_vacuum.c
@@ -21,6 +21,8 @@
  */
 #include "columnar.h"
 #include "columnar_metadata.h"
+#include "columnar_storage.h"
+#include "columnar_write_state.h"
 #include "columnar_compat.h"
 
 #include "fmgr.h"
diff --git a/src/columnar_vector.c b/src/columnar_vector.c
index 2d009bb0..489ed5a7 100644
--- a/src/columnar_vector.c
+++ b/src/columnar_vector.c
@@ -36,6 +36,7 @@
  */
 #include "columnar.h"
 
+#include "columnar_customscan.h"
 #include "columnar_metadata.h"
 #include "columnar_reader.h"
 #include <math.h>
diff --git a/src/columnar_write_state.c b/src/columnar_write_state.c
index 12a99870..cc773d20 100644
--- a/src/columnar_write_state.c
+++ b/src/columnar_write_state.c
@@ -14,6 +14,8 @@
 #include "columnar.h"
 
 #include "columnar_metadata.h"
+#include "columnar_storage.h"
+#include "columnar_write_state.h"
 #include "columnar_compat.h"
 #include "access/htup_details.h"
 #include "access/table.h"
diff --git a/src/columnar_write_state.h b/src/columnar_write_state.h
new file mode 100644
index 00000000..91d12651
--- /dev/null
+++ b/src/columnar_write_state.h
@@ -0,0 +1,47 @@
+/*-------------------------------------------------------------------------
+ *
+ * columnar_write_state.h
+ *		The write side: what columnar_write_state.c offers the modules that stage and flush rows.
+ *
+ * Split out of columnar.h (#496). Every declaration here had exactly ONE
+ * consumer outside its defining file, so it was a private arrangement between
+ * two files that the other twenty were forced to recompile for.
+ *
+ * The shared vocabulary these signatures take stays in columnar.h, which this
+ * includes.
+ *
+ * Written fresh for pgColumnar.
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PGCOLUMNAR_WRITE_STATE_H
+#define PGCOLUMNAR_WRITE_STATE_H
+
+#include "columnar.h"
+
+extern void PgColumnarEnsureStorageRow(Relation rel);	/* pre-create storage row (#300 parallel_copy) */
+
+extern int PgColumnarWriteStateStripeCount(PgColumnarWriteState *ws);
+
+extern uint64 *PgColumnarWriteStateStripeIds(PgColumnarWriteState *ws, int *n);
+
+extern uint64 *PgColumnarWriteStateProjStripeIds(PgColumnarWriteState *ws, int *n);
+
+extern void PgColumnarBackfillProjection(Relation rel,
+									   const PgColumnarProjection *proj);
+
+extern bool PgColumnarBufferedRowByNumber(Relation rel, uint64 rowNumber,
+										Datum *values, bool *nulls);
+
+extern void PgColumnarForgetWriteStateForRelation(Oid relid);
+
+extern void PgColumnarFlushAllPendingWrites(void);
+
+extern void PgColumnarDiscardAllPendingWrites(void);
+
+extern void PgColumnarWriteStateDiscardSubXact(SubTransactionId subid);
+
+extern void PgColumnarWriteStatePromoteSubXact(SubTransactionId subid,
+											 SubTransactionId parent);
+
+#endif							/* PGCOLUMNAR_WRITE_STATE_H */