granicus.if.org Git - postgresql/blob - src/include/utils/pg_lzcompress.h

   1 /* ----------
   2  * pg_lzcompress.h -
   3  *
   4  *      Definitions for the builtin LZ compressor
   5  *
   6  * src/include/utils/pg_lzcompress.h
   7  * ----------
   8  */
   9
  10 #ifndef _PG_LZCOMPRESS_H_
  11 #define _PG_LZCOMPRESS_H_
  12
  13
  14 /* ----------
  15  * PGLZ_Header -
  16  *
  17  *              The information at the start of the compressed data.
  18  * ----------
  19  */
  20 typedef struct PGLZ_Header
  21 {
  22         int32           vl_len_;                /* varlena header (do not touch directly!) */
  23         int32           rawsize;
  24 } PGLZ_Header;
  25
  26
  27 /* ----------
  28  * PGLZ_MAX_OUTPUT -
  29  *
  30  *              Macro to compute the buffer size required by pglz_compress().
  31  *              We allow 4 bytes for overrun before detecting compression failure.
  32  * ----------
  33  */
  34 #define PGLZ_MAX_OUTPUT(_dlen)                  ((_dlen) + 4 + sizeof(PGLZ_Header))
  35
  36 /* ----------
  37  * PGLZ_RAW_SIZE -
  38  *
  39  *              Macro to determine the uncompressed data size contained
  40  *              in the entry.
  41  * ----------
  42  */
  43 #define PGLZ_RAW_SIZE(_lzdata)                  ((_lzdata)->rawsize)
  44
  45
  46 /* ----------
  47  * PGLZ_Strategy -
  48  *
  49  *              Some values that control the compression algorithm.
  50  *
  51  *              min_input_size          Minimum input data size to consider compression.
  52  *
  53  *              max_input_size          Maximum input data size to consider compression.
  54  *
  55  *              min_comp_rate           Minimum compression rate (0-99%) to require.
  56  *                                                      Regardless of min_comp_rate, the output must be
  57  *                                                      smaller than the input, else we don't store
  58  *                                                      compressed.
  59  *
  60  *              first_success_by        Abandon compression if we find no compressible
  61  *                                                      data within the first this-many bytes.
  62  *
  63  *              match_size_good         The initial GOOD match size when starting history
  64  *                                                      lookup. When looking up the history to find a
  65  *                                                      match that could be expressed as a tag, the
  66  *                                                      algorithm does not always walk back entirely.
  67  *                                                      A good match fast is usually better than the
  68  *                                                      best possible one very late. For each iteration
  69  *                                                      in the lookup, this value is lowered so the
  70  *                                                      longer the lookup takes, the smaller matches
  71  *                                                      are considered good.
  72  *
  73  *              match_size_drop         The percentage by which match_size_good is lowered
  74  *                                                      after each history check. Allowed values are
  75  *                                                      0 (no change until end) to 100 (only check
  76  *                                                      latest history entry at all).
  77  * ----------
  78  */
  79 typedef struct PGLZ_Strategy
  80 {
  81         int32           min_input_size;
  82         int32           max_input_size;
  83         int32           min_comp_rate;
  84         int32           first_success_by;
  85         int32           match_size_good;
  86         int32           match_size_drop;
  87 } PGLZ_Strategy;
  88
  89
  90 /* ----------
  91  * The standard strategies
  92  *
  93  *              PGLZ_strategy_default           Recommended default strategy for TOAST.
  94  *
  95  *              PGLZ_strategy_always            Try to compress inputs of any length.
  96  *                                                                      Fallback to uncompressed storage only if
  97  *                                                                      output would be larger than input.
  98  * ----------
  99  */
 100 extern const PGLZ_Strategy *const PGLZ_strategy_default;
 101 extern const PGLZ_Strategy *const PGLZ_strategy_always;
 102
 103
 104 /* ----------
 105  * Global function declarations
 106  * ----------
 107  */
 108 extern bool pglz_compress(const char *source, int32 slen, PGLZ_Header *dest,
 109                           const PGLZ_Strategy *strategy);
 110 extern void pglz_decompress(const PGLZ_Header *source, char *dest);
 111
 112 #endif   /* _PG_LZCOMPRESS_H_ */