/* ----------
* pg_lzcompress.h -
*
* Definitions for the builtin LZ compressor
*
* $PostgreSQL: pgsql/src/include/utils/pg_lzcompress.h,v 1.17 2008/03/07 23:20:21 tgl Exp $
* ----------
*/
#ifndef _PG_LZCOMPRESS_H_
#define _PG_LZCOMPRESS_H_
/* ----------
* PGLZ_Header -
*
* The information at the start of the compressed data.
* ----------
*/
typedef struct PGLZ_Header
{
int32 vl_len_; /* varlena header (do not touch directly!) */
int32 rawsize;
} PGLZ_Header;
/* ----------
* PGLZ_MAX_OUTPUT -
*
* Macro to compute the buffer size required by pglz_compress().
* We allow 4 bytes for overrun before detecting compression failure.
* ----------
*/
#define PGLZ_MAX_OUTPUT(_dlen) ((_dlen) + 4 + sizeof(PGLZ_Header))
/* ----------
* PGLZ_RAW_SIZE -
*
* Macro to determine the uncompressed data size contained
* in the entry.
* ----------
*/
#define PGLZ_RAW_SIZE(_lzdata) ((_lzdata)->rawsize)
/* ----------
* PGLZ_Strategy -
*
* Some values that control the compression algorithm.
*
* min_input_size Minimum input data size to consider compression.
*
* max_input_size Maximum input data size to consider compression.
*
* min_comp_rate Minimum compression rate (0-99%) to require.
* Regardless of min_comp_rate, the output must be
* smaller than the input, else we don't store
* compressed.
*
* first_success_by Abandon compression if we find no compressible
* data within the first this-many bytes.
*
* match_size_good The initial GOOD match size when starting history
* lookup. When looking up the history to find a
* match that could be expressed as a tag, the
* algorithm does not always walk back entirely.
* A good match fast is usually better than the
* best possible one very late. For each iteration
* in the lookup, this value is lowered so the
* longer the lookup takes, the smaller matches
* are considered good.
*
* match_size_drop The percentage by which match_size_good is lowered
* after each history check. Allowed values are
* 0 (no change until end) to 100 (only check
* latest history entry at all).
* ----------
*/
typedef struct PGLZ_Strategy
{
int32 min_input_size;
int32 max_input_size;
int32 min_comp_rate;
int32 first_success_by;
int32 match_size_good;
int32 match_size_drop;
} PGLZ_Strategy;
/* ----------
* The standard strategies
*
* PGLZ_strategy_default Recommended default strategy for TOAST.
*
* PGLZ_strategy_always Try to compress inputs of any length.
* Fallback to uncompressed storage only if
* output would be larger than input.
* ----------
*/
extern const PGLZ_Strategy *const PGLZ_strategy_default;
extern const PGLZ_Strategy *const PGLZ_strategy_always;
/* ----------
* Global function declarations
* ----------
*/
extern bool pglz_compress(const char *source, int32 slen, PGLZ_Header *dest,
const PGLZ_Strategy *strategy);
extern void pglz_decompress(const PGLZ_Header *source, char *dest);
#endif /* _PG_LZCOMPRESS_H_ */
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