sql_expression_cache.cc

/* Copyright (C) 2010-2011 Monty Program Ab & Oleksandr Byelkin

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; version 2 of the License.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA */

#include "mariadb.h"
#include "sql_base.h"
#include "sql_select.h"
#include "sql_expression_cache.h"

/**
  Minimum hit ration to proceed on disk if in memory table overflowed.
  hit_rate = hit / (miss + hit);
*/
#define EXPCACHE_MIN_HIT_RATE_FOR_DISK_TABLE 0.7
/**
  Minimum hit ratio to keep in memory table (do not switch cache off)
  hit_rate = hit / (miss + hit);
*/
#define EXPCACHE_MIN_HIT_RATE_FOR_MEM_TABLE  0.2
/**
  Number of cache miss to check hit ratio (maximum cache performance
  impact in the case when the cache is not applicable)
*/
#define EXPCACHE_CHECK_HIT_RATIO_AFTER 200

/*
  Expression cache is used only for caching subqueries now, so its statistic
  variables we call subquery_cache*.
*/
ulong subquery_cache_miss, subquery_cache_hit;

Expression_cache_tmptable::Expression_cache_tmptable(THD *thd,
                                                     List<Item> &dependants,
                                                     Item *value)
  :cache_table(NULL), table_thd(thd), tracker(NULL), items(dependants), val(value),
   hit(0), miss(0), inited (0)
{
  DBUG_ENTER("Expression_cache_tmptable::Expression_cache_tmptable");
  DBUG_VOID_RETURN;
};


/**
  Disable cache
*/

void Expression_cache_tmptable::disable_cache()
{
  if (cache_table->file->inited)
    cache_table->file->ha_index_end();
  free_tmp_table(table_thd, cache_table);
  cache_table= NULL;
  update_tracker();
  if (tracker)
    tracker->detach_from_cache();
}


/**
  Field enumerator for TABLE::add_tmp_key

  @param arg             reference variable with current field number

  @return field number
*/

static uint field_enumerator(uchar *arg)
{
  return ((uint*)arg)[0]++;
}


/**
  Initialize temporary table and auxiliary structures for the expression
  cache

  @details
  The function creates a temporary table for the expression cache, defines
  the search index and initializes auxiliary search structures used to check
  whether a given set of of values of the expression parameters is in some
  cache entry.
*/

void Expression_cache_tmptable::init()
{
  List_iterator<Item> li(items);
  Item_iterator_list it(li);
  uint field_counter;
  LEX_CSTRING cache_table_name= { STRING_WITH_LEN("subquery-cache-table") };
  DBUG_ENTER("Expression_cache_tmptable::init");
  DBUG_ASSERT(!inited);
  inited= TRUE;
  cache_table= NULL;

  if (items.elements == 0)
  {
    DBUG_PRINT("info", ("All parameters were removed by optimizer."));
    DBUG_VOID_RETURN;
  }

  /* add result field */
  items.push_front(val);

  cache_table_param.init();
  /* dependent items and result */
  cache_table_param.field_count= cache_table_param.func_count= items.elements;
  /* postpone table creation to index description */
  cache_table_param.skip_create_table= 1;

  if (!(cache_table= create_tmp_table(table_thd, &cache_table_param,
                                      items, (ORDER*) NULL,
                                      FALSE, TRUE,
                                      ((table_thd->variables.option_bits |
                                        TMP_TABLE_ALL_COLUMNS) &
                                        ~TMP_TABLE_FORCE_MYISAM),
                                      HA_POS_ERROR,
                                      &cache_table_name,
                                      TRUE)))
  {
    DBUG_PRINT("error", ("create_tmp_table failed, caching switched off"));
    DBUG_VOID_RETURN;
  }

  if (cache_table->s->db_type() != heap_hton)
  {
    DBUG_PRINT("error", ("we need only heap table"));
    goto error;
  }

  field_counter= 1;

  if (cache_table->alloc_keys(1) ||
      cache_table->add_tmp_key(0, items.elements - 1, &field_enumerator,
                                (uchar*)&field_counter, TRUE) ||
      ref.tmp_table_index_lookup_init(table_thd, cache_table->key_info, it,
                                      TRUE, 1 /* skip result field*/))
  {
    DBUG_PRINT("error", ("creating index failed"));
    goto error;
  }
  cache_table->s->keys= 1;
  ref.null_rejecting= 1;
  ref.const_ref_part_map= 0;
  ref.disable_cache= FALSE;
  ref.has_record= 0;
  ref.use_count= 0;


  if (open_tmp_table(cache_table))
  {
    DBUG_PRINT("error", ("Opening (creating) temporary table failed"));
    goto error;
  }

  if (!(cached_result= new (table_thd->mem_root)
        Item_field(table_thd, cache_table->field[0])))
  {
    DBUG_PRINT("error", ("Creating Item_field failed"));
    goto error;
  }

  update_tracker();
  DBUG_VOID_RETURN;

error:
  disable_cache();
  DBUG_VOID_RETURN;
}


Expression_cache_tmptable::~Expression_cache_tmptable()
{
  /* Add accumulated statistics */
  statistic_add(subquery_cache_miss, miss, &LOCK_status);
  statistic_add(subquery_cache_hit, hit, &LOCK_status);

  if (cache_table)
    disable_cache();
  else
  {
    update_tracker();
    if (tracker)
      tracker->detach_from_cache();
    tracker= NULL;
  }
}


/**
  Check if a given set of parameters of the expression is in the cache

  @param [out] value     the expression value found in the cache if any

  @details
  For a given set of the parameters of the expression the function
  checks whether it can be found in some entry of the cache. If so
  the function returns the result of the expression extracted from
  the cache.

  @retval Expression_cache::HIT if the set of parameters is in the cache
  @retval Expression_cache::MISS - otherwise
*/

Expression_cache::result Expression_cache_tmptable::check_value(Item **value)
{
  int res;
  DBUG_ENTER("Expression_cache_tmptable::check_value");

  if (cache_table)
  {
    DBUG_PRINT("info", ("status: %u  has_record %u",
                        (uint)cache_table->status, (uint)ref.has_record));
    if ((res= join_read_key2(table_thd, NULL, cache_table, &ref)) == 1)
      DBUG_RETURN(ERROR);

    if (res)
    {
      if (((++miss) == EXPCACHE_CHECK_HIT_RATIO_AFTER) &&
          ((double)hit / ((double)hit + miss)) <
          EXPCACHE_MIN_HIT_RATE_FOR_MEM_TABLE)
      {
        DBUG_PRINT("info",
                   ("Early check: hit rate is not so good to keep the cache"));
        disable_cache();
      }

      DBUG_RETURN(MISS);
    }

    hit++;
    *value= cached_result;
    DBUG_RETURN(Expression_cache::HIT);
  }
  DBUG_RETURN(Expression_cache::MISS);
}


/**
  Put a new entry into the expression cache

  @param value     the result of the expression to be put into the cache

  @details
  The function evaluates 'value' and puts the result into the cache as the
  result of the expression for the current set of parameters.

  @retval FALSE OK
  @retval TRUE  Error
*/

my_bool Expression_cache_tmptable::put_value(Item *value)
{
  int error;
  DBUG_ENTER("Expression_cache_tmptable::put_value");
  DBUG_ASSERT(inited);

  if (!cache_table)
  {
    DBUG_PRINT("info", ("No table so behave as we successfully put value"));
    DBUG_RETURN(FALSE);
  }

  *(items.head_ref())= value;
  fill_record(table_thd, cache_table, cache_table->field, items, TRUE, TRUE);
  if (unlikely(table_thd->is_error()))
    goto err;;

  if (unlikely((error=
                cache_table->file->ha_write_tmp_row(cache_table->record[0]))))
  {
    /* create_myisam_from_heap will generate error if needed */
    if (cache_table->file->is_fatal_error(error, HA_CHECK_DUP))
      goto err;
    else
    {
      double hit_rate= ((double)hit / ((double)hit + miss));
      DBUG_ASSERT(miss > 0);
      if (hit_rate < EXPCACHE_MIN_HIT_RATE_FOR_MEM_TABLE)
      {
        DBUG_PRINT("info", ("hit rate is not so good to keep the cache"));
        disable_cache();
        DBUG_RETURN(FALSE);
      }
      else if (hit_rate < EXPCACHE_MIN_HIT_RATE_FOR_DISK_TABLE)
      {
        DBUG_PRINT("info", ("hit rate is not so good to go to disk"));
        if (cache_table->file->ha_delete_all_rows() ||
            cache_table->file->ha_write_tmp_row(cache_table->record[0]))
          goto err;
      }
      else
      {
        if (create_internal_tmp_table_from_heap(table_thd, cache_table,
                                                cache_table_param.start_recinfo,
                                                &cache_table_param.recinfo,
                                                error, 1, NULL))
          goto err;
      }
    }
  }
  cache_table->status= 0; /* cache_table->record contains an existed record */
  ref.has_record= TRUE; /* the same as above */
  DBUG_PRINT("info", ("has_record: TRUE  status: 0"));

  DBUG_RETURN(FALSE);

err:
  disable_cache();
  DBUG_RETURN(TRUE);
}


void Expression_cache_tmptable::print(String *str, enum_query_type query_type)
{
  List_iterator<Item> li(items);
  Item *item;
  bool is_first= TRUE;

  str->append('<');
  li++;  // skip result field
  while ((item= li++))
  {
    if (!is_first)
      str->append(',');
    item->print(str, query_type);
    is_first= FALSE;
  }
  str->append('>');
}


const char *Expression_cache_tracker::state_str[3]=
{"uninitialized", "disabled", "enabled"};