src/include/access/toast_compression.h

/*-------------------------------------------------------------------------
 *
 * toast_compression.h
 *	  Functions for toast compression.
 *
 * Copyright (c) 2021, PostgreSQL Global Development Group
 *
 * src/include/access/toast_compression.h
 *
 *-------------------------------------------------------------------------
 */

#ifndef TOAST_COMPRESSION_H
#define TOAST_COMPRESSION_H

/*
 * GUC support.
 *
 * default_toast_compression is an integer for purposes of the GUC machinery,
 * but the value is one of the char values defined below, as they appear in
 * pg_attribute.attcompression, e.g. TOAST_PGLZ_COMPRESSION.
 */
extern int	default_toast_compression;

/*
 * Built-in compression method ID.  The toast compression header will store
 * this in the first 2 bits of the raw length.  These built-in compression
 * method IDs are directly mapped to the built-in compression methods.
 *
 * Don't use these values for anything other than understanding the meaning
 * of the raw bits from a varlena; in particular, if the goal is to identify
 * a compression method, use the constants TOAST_PGLZ_COMPRESSION, etc.
 * below. We might someday support more than 4 compression methods, but
 * we can never have more than 4 values in this enum, because there are
 * only 2 bits available in the places where this is stored.
 */
typedef enum ToastCompressionId
{
	TOAST_PGLZ_COMPRESSION_ID = 0,
	TOAST_LZ4_COMPRESSION_ID = 1,
	TOAST_INVALID_COMPRESSION_ID = 2
} ToastCompressionId;

/*
 * Built-in compression methods.  pg_attribute will store these in the
 * attcompression column.  In attcompression, InvalidCompressionMethod
 * denotes the default behavior.
 */
#define TOAST_PGLZ_COMPRESSION			'p'
#define TOAST_LZ4_COMPRESSION			'l'
#define InvalidCompressionMethod		'\0'

#define CompressionMethodIsValid(cm)  ((cm) != InvalidCompressionMethod)


/* pglz compression/decompression routines */
extern struct varlena *pglz_compress_datum(const struct varlena *value);
extern struct varlena *pglz_decompress_datum(const struct varlena *value);
extern struct varlena *pglz_decompress_datum_slice(const struct varlena *value,
												   int32 slicelength);

/* lz4 compression/decompression routines */
extern struct varlena *lz4_compress_datum(const struct varlena *value);
extern struct varlena *lz4_decompress_datum(const struct varlena *value);
extern struct varlena *lz4_decompress_datum_slice(const struct varlena *value,
												  int32 slicelength);

/* other stuff */
extern ToastCompressionId toast_get_compression_id(struct varlena *attr);
extern char CompressionNameToMethod(const char *compression);
extern const char *GetCompressionMethodName(char method);

#endif							/* TOAST_COMPRESSION_H */