#include "sql_parse.h" #include "sql_select.h" /**************************************************************************** * Default MRR implementation (MRR to non-MRR converter) ***************************************************************************/ /** Get cost and other information about MRR scan over a known list of ranges Calculate estimated cost and other information about an MRR scan for given sequence of ranges. @param keyno Index number @param seq Range sequence to be traversed @param seq_init_param First parameter for seq->init() @param n_ranges_arg Number of ranges in the sequence, or 0 if the caller can't efficiently determine it @param bufsz INOUT IN: Size of the buffer available for use OUT: Size of the buffer that is expected to be actually used, or 0 if buffer is not needed. @param flags INOUT A combination of HA_MRR_* flags @param cost OUT Estimated cost of MRR access @note This method (or an overriding one in a derived class) must check for thd->killed and return HA_POS_ERROR if it is not zero. This is required for a user to be able to interrupt the calculation by killing the connection/query. @retval HA_POS_ERROR Error or the engine is unable to perform the requested scan. Values of OUT parameters are undefined. @retval other OK, *cost contains cost of the scan, *bufsz and *flags contain scan parameters. */ ha_rows handler::multi_range_read_info_const(uint keyno, RANGE_SEQ_IF *seq, void *seq_init_param, uint n_ranges_arg, uint *bufsz, uint *flags, COST_VECT *cost) { KEY_MULTI_RANGE range; range_seq_t seq_it; ha_rows rows, total_rows= 0; uint n_ranges=0; THD *thd= current_thd; /* Default MRR implementation doesn't need buffer */ *bufsz= 0; seq_it= seq->init(seq_init_param, n_ranges, *flags); while (!seq->next(seq_it, &range)) { if (unlikely(thd->killed != 0)) return HA_POS_ERROR; n_ranges++; key_range *min_endp, *max_endp; if (range.range_flag & GEOM_FLAG) { /* In this case tmp_min_flag contains the handler-read-function */ range.start_key.flag= (ha_rkey_function) (range.range_flag ^ GEOM_FLAG); min_endp= &range.start_key; max_endp= NULL; } else { min_endp= range.start_key.length? &range.start_key : NULL; max_endp= range.end_key.length? &range.end_key : NULL; } if ((range.range_flag & UNIQUE_RANGE) && !(range.range_flag & NULL_RANGE)) rows= 1; /* there can be at most one row */ else { if (HA_POS_ERROR == (rows= this->records_in_range(keyno, min_endp, max_endp))) { /* Can't scan one range => can't do MRR scan at all */ total_rows= HA_POS_ERROR; break; } } total_rows += rows; } if (total_rows != HA_POS_ERROR) { /* The following calculation is the same as in multi_range_read_info(): */ *flags |= HA_MRR_USE_DEFAULT_IMPL; cost->zero(); cost->avg_io_cost= 1; /* assume random seeks */ if ((*flags & HA_MRR_INDEX_ONLY) && total_rows > 2) cost->io_count= keyread_time(keyno, n_ranges, (uint)total_rows); else cost->io_count= read_time(keyno, n_ranges, total_rows); cost->cpu_cost= (double) total_rows / TIME_FOR_COMPARE + 0.01; } return total_rows; } /** Get cost and other information about MRR scan over some sequence of ranges Calculate estimated cost and other information about an MRR scan for some sequence of ranges. The ranges themselves will be known only at execution phase. When this function is called we only know number of ranges and a (rough) E(#records) within those ranges. Currently this function is only called for "n-keypart singlepoint" ranges, i.e. each range is "keypart1=someconst1 AND ... AND keypartN=someconstN" The flags parameter is a combination of those flags: HA_MRR_SORTED, HA_MRR_INDEX_ONLY, HA_MRR_NO_ASSOCIATION, HA_MRR_LIMITS. @param keyno Index number @param n_ranges Estimated number of ranges (i.e. intervals) in the range sequence. @param n_rows Estimated total number of records contained within all of the ranges @param bufsz INOUT IN: Size of the buffer available for use OUT: Size of the buffer that will be actually used, or 0 if buffer is not needed. @param flags INOUT A combination of HA_MRR_* flags @param cost OUT Estimated cost of MRR access @retval 0 OK, *cost contains cost of the scan, *bufsz and *flags contain scan parameters. @retval other Error or can't perform the requested scan */ ha_rows handler::multi_range_read_info(uint keyno, uint n_ranges, uint n_rows, uint *bufsz, uint *flags, COST_VECT *cost) { *bufsz= 0; /* Default implementation doesn't need a buffer */ *flags |= HA_MRR_USE_DEFAULT_IMPL; cost->zero(); cost->avg_io_cost= 1; /* assume random seeks */ /* Produce the same cost as non-MRR code does */ if (*flags & HA_MRR_INDEX_ONLY) cost->io_count= keyread_time(keyno, n_ranges, n_rows); else cost->io_count= read_time(keyno, n_ranges, n_rows); return 0; } /** Initialize the MRR scan Initialize the MRR scan. This function may do heavyweight scan initialization like row prefetching/sorting/etc (NOTE: but better not do it here as we may not need it, e.g. if we never satisfy WHERE clause on previous tables. For many implementations it would be natural to do such initializations in the first multi_read_range_next() call) mode is a combination of the following flags: HA_MRR_SORTED, HA_MRR_INDEX_ONLY, HA_MRR_NO_ASSOCIATION @param seq Range sequence to be traversed @param seq_init_param First parameter for seq->init() @param n_ranges Number of ranges in the sequence @param mode Flags, see the description section for the details @param buf INOUT: memory buffer to be used @note One must have called index_init() before calling this function. Several multi_range_read_init() calls may be made in course of one query. Until WL#2623 is done (see its text, section 3.2), the following will also hold: The caller will guarantee that if "seq->init == mrr_ranges_array_init" then seq_init_param is an array of n_ranges KEY_MULTI_RANGE structures. This property will only be used by NDB handler until WL#2623 is done. Buffer memory management is done according to the following scenario: The caller allocates the buffer and provides it to the callee by filling the members of HANDLER_BUFFER structure. The callee consumes all or some fraction of the provided buffer space, and sets the HANDLER_BUFFER members accordingly. The callee may use the buffer memory until the next multi_range_read_init() call is made, all records have been read, or until index_end() call is made, whichever comes first. @retval 0 OK @retval 1 Error */ int handler::multi_range_read_init(RANGE_SEQ_IF *seq_funcs, void *seq_init_param, uint n_ranges, uint mode, HANDLER_BUFFER *buf) { DBUG_ENTER("handler::multi_range_read_init"); mrr_iter= seq_funcs->init(seq_init_param, n_ranges, mode); mrr_funcs= *seq_funcs; mrr_is_output_sorted= test(mode & HA_MRR_SORTED); mrr_have_range= FALSE; DBUG_RETURN(0); } /** Get next record in MRR scan Default MRR implementation: read the next record @param range_info OUT Undefined if HA_MRR_NO_ASSOCIATION flag is in effect Otherwise, the opaque value associated with the range that contains the returned record. @retval 0 OK @retval other Error code */ int handler::multi_range_read_next(char **range_info) { int UNINIT_VAR(result); int range_res; DBUG_ENTER("handler::multi_range_read_next"); if (!mrr_have_range) { mrr_have_range= TRUE; goto start; } do { /* Save a call if there can be only one row in range. */ if (mrr_cur_range.range_flag != (UNIQUE_RANGE | EQ_RANGE)) { result= read_range_next(); /* On success or non-EOF errors jump to the end. */ if (result != HA_ERR_END_OF_FILE) break; } else { if (was_semi_consistent_read()) goto scan_it_again; /* We need to set this for the last range only, but checking this condition is more expensive than just setting the result code. */ result= HA_ERR_END_OF_FILE; } start: /* Try the next range(s) until one matches a record. */ while (!(range_res= mrr_funcs.next(mrr_iter, &mrr_cur_range))) { scan_it_again: result= read_range_first(mrr_cur_range.start_key.keypart_map ? &mrr_cur_range.start_key : 0, mrr_cur_range.end_key.keypart_map ? &mrr_cur_range.end_key : 0, test(mrr_cur_range.range_flag & EQ_RANGE), mrr_is_output_sorted); if (result != HA_ERR_END_OF_FILE) break; } } while ((result == HA_ERR_END_OF_FILE) && !range_res); *range_info= mrr_cur_range.ptr; DBUG_PRINT("exit",("handler::multi_range_read_next result %d", result)); DBUG_RETURN(result); } /**************************************************************************** * DS-MRR implementation ***************************************************************************/ /** DS-MRR: Initialize and start MRR scan Initialize and start the MRR scan. Depending on the mode parameter, this may use default or DS-MRR implementation. @param h Table handler to be used @param key Index to be used @param seq_funcs Interval sequence enumeration functions @param seq_init_param Interval sequence enumeration parameter @param n_ranges Number of ranges in the sequence. @param mode HA_MRR_* modes to use @param buf INOUT Buffer to use @retval 0 Ok, Scan started. @retval other Error */ int DsMrr_impl::dsmrr_init(handler *h_arg, RANGE_SEQ_IF *seq_funcs, void *seq_init_param, uint n_ranges, uint mode, HANDLER_BUFFER *buf) { uint elem_size; Item *pushed_cond= NULL; handler *new_h2= 0; DBUG_ENTER("DsMrr_impl::dsmrr_init"); /* index_merge may invoke a scan on an object for which dsmrr_info[_const] has not been called, so set the owner handler here as well. */ h= h_arg; if (mode & HA_MRR_USE_DEFAULT_IMPL || mode & HA_MRR_SORTED) { use_default_impl= TRUE; const int retval= h->handler::multi_range_read_init(seq_funcs, seq_init_param, n_ranges, mode, buf); DBUG_RETURN(retval); } rowids_buf= buf->buffer; is_mrr_assoc= !test(mode & HA_MRR_NO_ASSOCIATION); if (is_mrr_assoc) status_var_increment(table->in_use->status_var.ha_multi_range_read_init_count); rowids_buf_end= buf->buffer_end; elem_size= h->ref_length + (int)is_mrr_assoc * sizeof(void*); rowids_buf_last= rowids_buf + ((rowids_buf_end - rowids_buf)/ elem_size)* elem_size; rowids_buf_end= rowids_buf_last; /* There can be two cases: - This is the first call since index_init(), h2==NULL Need to setup h2 then. - This is not the first call, h2 is initalized and set up appropriately. The caller might have called h->index_init(), need to switch h to rnd_pos calls. */ if (!h2) { /* Create a separate handler object to do rndpos() calls. */ THD *thd= current_thd; /* ::clone() takes up a lot of stack, especially on 64 bit platforms. The constant 5 is an empiric result. */ if (check_stack_overrun(thd, 5*STACK_MIN_SIZE, (uchar*) &new_h2)) DBUG_RETURN(1); DBUG_ASSERT(h->active_index != MAX_KEY); uint mrr_keyno= h->active_index; /* Create a separate handler object to do rndpos() calls. */ if (!(new_h2= h->clone(h->table->s->normalized_path.str, thd->mem_root)) || new_h2->ha_external_lock(thd, F_RDLCK)) { delete new_h2; DBUG_RETURN(1); } if (mrr_keyno == h->pushed_idx_cond_keyno) pushed_cond= h->pushed_idx_cond; /* Caution: this call will invoke this->dsmrr_close(). Do not put the created secondary table handler into this->h2 or it will delete it. */ if (h->ha_index_end()) { h2=new_h2; goto error; } h2= new_h2; /* Ok, now can put it into h2 */ table->prepare_for_position(); h2->extra(HA_EXTRA_KEYREAD); if (h2->ha_index_init(mrr_keyno, FALSE)) goto error; use_default_impl= FALSE; if (pushed_cond) h2->idx_cond_push(mrr_keyno, pushed_cond); } else { /* We get here when the access alternates betwen MRR scan(s) and non-MRR scans. Calling h->index_end() will invoke dsmrr_close() for this object, which will delete h2. We need to keep it, so save put it away and dont let it be deleted: */ handler *save_h2= h2; h2= NULL; int res= (h->inited == handler::INDEX && h->ha_index_end()); h2= save_h2; use_default_impl= FALSE; if (res) goto error; } if (h2->handler::multi_range_read_init(seq_funcs, seq_init_param, n_ranges, mode, buf) || dsmrr_fill_buffer()) { goto error; } /* If the above call has scanned through all intervals in *seq, then adjust *buf to indicate that the remaining buffer space will not be used. */ if (dsmrr_eof) buf->end_of_used_area= rowids_buf_last; /* h->inited == INDEX may occur when 'range checked for each record' is used. */ if ((h->inited != handler::RND) && ((h->inited==handler::INDEX? h->ha_index_end(): FALSE) || (h->ha_rnd_init(FALSE)))) goto error; use_default_impl= FALSE; h->mrr_funcs= *seq_funcs; DBUG_RETURN(0); error: h2->ha_index_or_rnd_end(); h2->ha_external_lock(current_thd, F_UNLCK); h2->close(); delete h2; h2= NULL; DBUG_RETURN(1); } void DsMrr_impl::dsmrr_close() { DBUG_ENTER("DsMrr_impl::dsmrr_close"); if (h2) { h2->ha_index_or_rnd_end(); h2->ha_external_lock(current_thd, F_UNLCK); h2->close(); delete h2; h2= NULL; } use_default_impl= TRUE; DBUG_VOID_RETURN; } static int rowid_cmp(void *h, uchar *a, uchar *b) { return ((handler*)h)->cmp_ref(a, b); } /** DS-MRR: Fill the buffer with rowids and sort it by rowid {This is an internal function of DiskSweep MRR implementation} Scan the MRR ranges and collect ROWIDs (or {ROWID, range_id} pairs) into buffer. When the buffer is full or scan is completed, sort the buffer by rowid and return. The function assumes that rowids buffer is empty when it is invoked. @param h Table handler @retval 0 OK, the next portion of rowids is in the buffer, properly ordered @retval other Error */ int DsMrr_impl::dsmrr_fill_buffer() { char *range_info; int res; DBUG_ENTER("DsMrr_impl::dsmrr_fill_buffer"); rowids_buf_cur= rowids_buf; while ((rowids_buf_cur < rowids_buf_end) && !(res= h2->handler::multi_range_read_next(&range_info))) { KEY_MULTI_RANGE *curr_range= &h2->handler::mrr_cur_range; if (h2->mrr_funcs.skip_index_tuple && h2->mrr_funcs.skip_index_tuple(h2->mrr_iter, curr_range->ptr)) continue; /* Put rowid, or {rowid, range_id} pair into the buffer */ h2->position(table->record[0]); memcpy(rowids_buf_cur, h2->ref, h2->ref_length); rowids_buf_cur += h2->ref_length; if (is_mrr_assoc) { memcpy(rowids_buf_cur, &range_info, sizeof(void*)); rowids_buf_cur += sizeof(void*); } } if (res && res != HA_ERR_END_OF_FILE) DBUG_RETURN(res); dsmrr_eof= test(res == HA_ERR_END_OF_FILE); /* Sort the buffer contents by rowid */ uint elem_size= h->ref_length + (int)is_mrr_assoc * sizeof(void*); uint n_rowids= (rowids_buf_cur - rowids_buf) / elem_size; my_qsort2(rowids_buf, n_rowids, elem_size, (qsort2_cmp)rowid_cmp, (void*)h); rowids_buf_last= rowids_buf_cur; rowids_buf_cur= rowids_buf; DBUG_RETURN(0); } /** DS-MRR implementation: multi_range_read_next() function */ int DsMrr_impl::dsmrr_next(char **range_info) { int res; uchar *cur_range_info= 0; uchar *rowid; if (use_default_impl) return h->handler::multi_range_read_next(range_info); do { if (rowids_buf_cur == rowids_buf_last) { if (dsmrr_eof) { res= HA_ERR_END_OF_FILE; goto end; } res= dsmrr_fill_buffer(); if (res) goto end; } /* return eof if there are no rowids in the buffer after re-fill attempt */ if (rowids_buf_cur == rowids_buf_last) { res= HA_ERR_END_OF_FILE; goto end; } rowid= rowids_buf_cur; if (is_mrr_assoc) memcpy(&cur_range_info, rowids_buf_cur + h->ref_length, sizeof(uchar**)); rowids_buf_cur += h->ref_length + sizeof(void*) * test(is_mrr_assoc); if (h2->mrr_funcs.skip_record && h2->mrr_funcs.skip_record(h2->mrr_iter, (char *) cur_range_info, rowid)) continue; res= h->ha_rnd_pos(table->record[0], rowid); break; } while (true); if (is_mrr_assoc) { memcpy(range_info, rowid + h->ref_length, sizeof(void*)); } end: return res; } /** DS-MRR implementation: multi_range_read_info() function */ ha_rows DsMrr_impl::dsmrr_info(uint keyno, uint n_ranges, uint rows, uint *bufsz, uint *flags, COST_VECT *cost) { ha_rows res; uint def_flags= *flags; uint def_bufsz= *bufsz; /* Get cost/flags/mem_usage of default MRR implementation */ res= h->handler::multi_range_read_info(keyno, n_ranges, rows, &def_bufsz, &def_flags, cost); DBUG_ASSERT(!res); if ((*flags & HA_MRR_USE_DEFAULT_IMPL) || choose_mrr_impl(keyno, rows, &def_flags, &def_bufsz, cost)) { /* Default implementation is choosen */ DBUG_PRINT("info", ("Default MRR implementation choosen")); *flags= def_flags; *bufsz= def_bufsz; } else { /* *flags and *bufsz were set by choose_mrr_impl */ DBUG_PRINT("info", ("DS-MRR implementation choosen")); } return 0; } /** DS-MRR Implementation: multi_range_read_info_const() function */ ha_rows DsMrr_impl::dsmrr_info_const(uint keyno, RANGE_SEQ_IF *seq, void *seq_init_param, uint n_ranges, uint *bufsz, uint *flags, COST_VECT *cost) { ha_rows rows; uint def_flags= *flags; uint def_bufsz= *bufsz; /* Get cost/flags/mem_usage of default MRR implementation */ rows= h->handler::multi_range_read_info_const(keyno, seq, seq_init_param, n_ranges, &def_bufsz, &def_flags, cost); if (rows == HA_POS_ERROR) { /* Default implementation can't perform MRR scan => we can't either */ return rows; } /* If HA_MRR_USE_DEFAULT_IMPL has been passed to us, that is an order to use the default MRR implementation (we need it for UPDATE/DELETE). Otherwise, make a choice based on cost and @@optimizer_use_mrr. */ if ((*flags & HA_MRR_USE_DEFAULT_IMPL) || choose_mrr_impl(keyno, rows, flags, bufsz, cost)) { DBUG_PRINT("info", ("Default MRR implementation choosen")); *flags= def_flags; *bufsz= def_bufsz; } else { /* *flags and *bufsz were set by choose_mrr_impl */ DBUG_PRINT("info", ("DS-MRR implementation choosen")); } return rows; } /** Check if key has partially-covered columns We can't use DS-MRR to perform range scans when the ranges are over partially-covered keys, because we'll not have full key part values (we'll have their prefixes from the index) and will not be able to check if we've reached the end the range. @param keyno Key to check @todo Allow use of DS-MRR in cases where the index has partially-covered components but they are not used for scanning. @retval TRUE Yes @retval FALSE No */ bool key_uses_partial_cols(TABLE *table, uint keyno) { KEY_PART_INFO *kp= table->key_info[keyno].key_part; KEY_PART_INFO *kp_end= kp + table->key_info[keyno].key_parts; for (; kp != kp_end; kp++) { if (!kp->field->part_of_key.is_set(keyno)) return TRUE; } return FALSE; } /** DS-MRR Internals: Choose between Default MRR implementation and DS-MRR Make the choice between using Default MRR implementation and DS-MRR. This function contains common functionality factored out of dsmrr_info() and dsmrr_info_const(). The function assumes that the default MRR implementation's applicability requirements are satisfied. @param keyno Index number @param rows E(full rows to be retrieved) @param flags IN MRR flags provided by the MRR user OUT If DS-MRR is choosen, flags of DS-MRR implementation else the value is not modified @param bufsz IN If DS-MRR is choosen, buffer use of DS-MRR implementation else the value is not modified @param cost IN Cost of default MRR implementation OUT If DS-MRR is choosen, cost of DS-MRR scan else the value is not modified @retval TRUE Default MRR implementation should be used @retval FALSE DS-MRR implementation should be used */ bool DsMrr_impl::choose_mrr_impl(uint keyno, ha_rows rows, uint *flags, uint *bufsz, COST_VECT *cost) { COST_VECT dsmrr_cost; bool res; THD *thd= current_thd; if (thd->variables.optimizer_use_mrr == 2 || *flags & HA_MRR_INDEX_ONLY || (keyno == table->s->primary_key && h->primary_key_is_clustered()) || key_uses_partial_cols(table, keyno)) { /* Use the default implementation */ *flags |= HA_MRR_USE_DEFAULT_IMPL; return TRUE; } uint add_len= table->key_info[keyno].key_length + h->ref_length; *bufsz -= add_len; if (get_disk_sweep_mrr_cost(keyno, rows, *flags, bufsz, &dsmrr_cost)) return TRUE; *bufsz += add_len; bool force_dsmrr; /* If @@optimizer_use_mrr==force, then set cost of DS-MRR to be minimum of DS-MRR and Default implementations cost. This allows one to force use of DS-MRR whenever it is applicable without affecting other cost-based choices. */ if ((force_dsmrr= (thd->variables.optimizer_use_mrr == 1)) && dsmrr_cost.total_cost() > cost->total_cost()) dsmrr_cost= *cost; if (force_dsmrr || dsmrr_cost.total_cost() <= cost->total_cost()) { *flags &= ~HA_MRR_USE_DEFAULT_IMPL; /* Use the DS-MRR implementation */ *flags &= ~HA_MRR_SORTED; /* We will return unordered output */ *cost= dsmrr_cost; res= FALSE; } else { /* Use the default MRR implementation */ res= TRUE; } return res; } static void get_sort_and_sweep_cost(TABLE *table, ha_rows nrows, COST_VECT *cost); /** Get cost of DS-MRR scan @param keynr Index to be used @param rows E(Number of rows to be scanned) @param flags Scan parameters (HA_MRR_* flags) @param buffer_size INOUT Buffer size @param cost OUT The cost @retval FALSE OK @retval TRUE Error, DS-MRR cannot be used (the buffer is too small for even 1 rowid) */ bool DsMrr_impl::get_disk_sweep_mrr_cost(uint keynr, ha_rows rows, uint flags, uint *buffer_size, COST_VECT *cost) { ulong max_buff_entries, elem_size; ha_rows rows_in_full_step, rows_in_last_step; uint n_full_steps; double index_read_cost; elem_size= h->ref_length + sizeof(void*) * (!test(flags & HA_MRR_NO_ASSOCIATION)); max_buff_entries = *buffer_size / elem_size; if (!max_buff_entries) return TRUE; /* Buffer has not enough space for even 1 rowid */ /* Number of iterations we'll make with full buffer */ n_full_steps= (uint)floor(rows2double(rows) / max_buff_entries); /* Get numbers of rows we'll be processing in - non-last sweep, with full buffer - last iteration, with non-full buffer */ rows_in_full_step= max_buff_entries; rows_in_last_step= rows % max_buff_entries; /* Adjust buffer size if we expect to use only part of the buffer */ if (n_full_steps) { get_sort_and_sweep_cost(table, rows, cost); cost->multiply(n_full_steps); } else { cost->zero(); *buffer_size= max(*buffer_size, (size_t)(1.2*rows_in_last_step) * elem_size + h->ref_length + table->key_info[keynr].key_length); } COST_VECT last_step_cost; get_sort_and_sweep_cost(table, rows_in_last_step, &last_step_cost); cost->add(&last_step_cost); if (n_full_steps != 0) cost->mem_cost= *buffer_size; else cost->mem_cost= (double)rows_in_last_step * elem_size; /* Total cost of all index accesses */ index_read_cost= h->keyread_time(keynr, 1, (double)rows); cost->add_io(index_read_cost, 1 /* Random seeks */); return FALSE; } /* Get cost of one sort-and-sweep step SYNOPSIS get_sort_and_sweep_cost() table Table being accessed nrows Number of rows to be sorted and retrieved cost OUT The cost DESCRIPTION Get cost of these operations: - sort an array of #nrows ROWIDs using qsort - read #nrows records from table in a sweep. */ static void get_sort_and_sweep_cost(TABLE *table, ha_rows nrows, COST_VECT *cost) { if (nrows) { get_sweep_read_cost(table, nrows, FALSE, cost); /* Add cost of qsort call: n * log2(n) * cost(rowid_comparison) */ double cmp_op= rows2double(nrows) * (1.0 / TIME_FOR_COMPARE_ROWID); if (cmp_op < 3) cmp_op= 3; cost->cpu_cost += cmp_op * log2(cmp_op); } else cost->zero(); } /** Get cost of reading nrows table records in a "disk sweep" A disk sweep read is a sequence of handler->rnd_pos(rowid) calls that made for an ordered sequence of rowids. We assume hard disk IO. The read is performed as follows: 1. The disk head is moved to the needed cylinder 2. The controller waits for the plate to rotate 3. The data is transferred Time to do #3 is insignificant compared to #2+#1. Time to move the disk head is proportional to head travel distance. Time to wait for the plate to rotate depends on whether the disk head was moved or not. If disk head wasn't moved, the wait time is proportional to distance between the previous block and the block we're reading. If the head was moved, we don't know how much we'll need to wait for the plate to rotate. We assume the wait time to be a variate with a mean of 0.5 of full rotation time. Our cost units are "random disk seeks". The cost of random disk seek is actually not a constant, it depends one range of cylinders we're going to access. We make it constant by introducing a fuzzy concept of "typical datafile length" (it's fuzzy as it's hard to tell whether it should include index file, temp.tables etc). Then random seek cost is: 1 = half_rotation_cost + move_cost * 1/3 * typical_data_file_length We define half_rotation_cost as DISK_SEEK_BASE_COST=0.9. @param table Table to be accessed @param nrows Number of rows to retrieve @param interrupted TRUE <=> Assume that the disk sweep will be interrupted by other disk IO. FALSE - otherwise. @param cost OUT The cost. */ void get_sweep_read_cost(TABLE *table, ha_rows nrows, bool interrupted, COST_VECT *cost) { DBUG_ENTER("get_sweep_read_cost"); cost->zero(); if (table->file->primary_key_is_clustered()) { cost->io_count= table->file->read_time(table->s->primary_key, (uint) nrows, nrows); } else { double n_blocks= ceil(ulonglong2double(table->file->stats.data_file_length) / IO_SIZE); double busy_blocks= n_blocks * (1.0 - pow(1.0 - 1.0/n_blocks, rows2double(nrows))); if (busy_blocks < 1.0) busy_blocks= 1.0; DBUG_PRINT("info",("sweep: nblocks=%g, busy_blocks=%g", n_blocks, busy_blocks)); cost->io_count= busy_blocks; if (!interrupted) { /* Assume reading is done in one 'sweep' */ cost->avg_io_cost= (DISK_SEEK_BASE_COST + DISK_SEEK_PROP_COST*n_blocks/busy_blocks); } } DBUG_PRINT("info",("returning cost=%g", cost->total_cost())); DBUG_VOID_RETURN; } /* ************************************************************************** * DS-MRR implementation ends ***************************************************************************/