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db_include/storage/large_object.h

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+/*-------------------------------------------------------------------------
+ *
+ * large_object.h
+ *	  Declarations for PostgreSQL large objects.  POSTGRES 4.2 supported
+ *	  zillions of large objects (internal, external, jaquith, inversion).
+ *	  Now we only support inversion.
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/storage/large_object.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef LARGE_OBJECT_H
+#define LARGE_OBJECT_H
+
+#include "utils/snapshot.h"
+
+
+/*----------
+ * Data about a currently-open large object.
+ *
+ * id is the logical OID of the large object
+ * snapshot is the snapshot to use for read/write operations
+ * subid is the subtransaction that opened the desc (or currently owns it)
+ * offset is the current seek offset within the LO
+ * flags contains some flag bits
+ *
+ * NOTE: as of v11, permission checks are made when the large object is
+ * opened; therefore IFS_RDLOCK/IFS_WRLOCK indicate that read or write mode
+ * has been requested *and* the corresponding permission has been checked.
+ *
+ * NOTE: before 7.1, we also had to store references to the separate table
+ * and index of a specific large object.  Now they all live in pg_largeobject
+ * and are accessed via a common relation descriptor.
+ *----------
+ */
+typedef struct LargeObjectDesc
+{
+	Oid			id;				/* LO's identifier */
+	Snapshot	snapshot;		/* snapshot to use */
+	SubTransactionId subid;		/* owning subtransaction ID */
+	uint64		offset;			/* current seek pointer */
+	int			flags;			/* see flag bits below */
+
+/* bits in flags: */
+#define IFS_RDLOCK		(1 << 0)	/* LO was opened for reading */
+#define IFS_WRLOCK		(1 << 1)	/* LO was opened for writing */
+
+} LargeObjectDesc;
+
+
+/*
+ * Each "page" (tuple) of a large object can hold this much data
+ *
+ * We could set this as high as BLCKSZ less some overhead, but it seems
+ * better to make it a smaller value, so that not as much space is used
+ * up when a page-tuple is updated.  Note that the value is deliberately
+ * chosen large enough to trigger the tuple toaster, so that we will
+ * attempt to compress page tuples in-line.  (But they won't be moved off
+ * unless the user creates a toast-table for pg_largeobject...)
+ *
+ * Also, it seems to be a smart move to make the page size be a power of 2,
+ * since clients will often be written to send data in power-of-2 blocks.
+ * This avoids unnecessary tuple updates caused by partial-page writes.
+ *
+ * NB: Changing LOBLKSIZE requires an initdb.
+ */
+#define LOBLKSIZE		(BLCKSZ / 4)
+
+/*
+ * Maximum length in bytes for a large object.  To make this larger, we'd
+ * have to widen pg_largeobject.pageno as well as various internal variables.
+ */
+#define MAX_LARGE_OBJECT_SIZE	((int64) INT_MAX * LOBLKSIZE)
+
+
+/*
+ * GUC: backwards-compatibility flag to suppress LO permission checks
+ */
+extern bool lo_compat_privileges;
+
+/*
+ * Function definitions...
+ */
+
+/* inversion stuff in inv_api.c */
+extern void close_lo_relation(bool isCommit);
+extern Oid	inv_create(Oid lobjId);
+extern LargeObjectDesc *inv_open(Oid lobjId, int flags, MemoryContext mcxt);
+extern void inv_close(LargeObjectDesc *obj_desc);
+extern int	inv_drop(Oid lobjId);
+extern int64 inv_seek(LargeObjectDesc *obj_desc, int64 offset, int whence);
+extern int64 inv_tell(LargeObjectDesc *obj_desc);
+extern int	inv_read(LargeObjectDesc *obj_desc, char *buf, int nbytes);
+extern int	inv_write(LargeObjectDesc *obj_desc, const char *buf, int nbytes);
+extern void inv_truncate(LargeObjectDesc *obj_desc, int64 len);
+
+#endif							/* LARGE_OBJECT_H */