/*
Get best plan for a SEL_IMERGE disjunctive expression.
SYNOPSIS
get_best_disjunct_quick()
param Parameter from check_quick_select function
imerge Expression to use
cost_est Don't create scans with cost > cost_est
NOTES
index_merge cost is calculated as follows:
index_merge_cost =
cost(index_reads) + (see #1)
cost(rowid_to_row_scan) + (see #2)
cost(unique_use) (see #3)
1. cost(index_reads) =SUM_i(cost(index_read_i))
For non-CPK scans,
cost(index_read_i) = {cost of ordinary 'index only' scan}
For CPK scan,
cost(index_read_i) = {cost of non-'index only' scan}
2. cost(rowid_to_row_scan)
If table PK is clustered then
cost(rowid_to_row_scan) =
{cost of ordinary clustered PK scan with n_ranges=n_rows}
Otherwise, we use the following model to calculate costs:
We need to retrieve n_rows rows from file that occupies n_blocks blocks.
We assume that offsets of rows we need are independent variates with
uniform distribution in [0..max_file_offset] range.
We'll denote block as "busy" if it contains row(s) we need to retrieve
and "empty" if doesn't contain rows we need.
Probability that a block is empty is (1 - 1/n_blocks)^n_rows (this
applies to any block in file). Let x_i be a variate taking value 1 if
block #i is empty and 0 otherwise.
Then E(x_i) = (1 - 1/n_blocks)^n_rows;
E(n_empty_blocks) = E(sum(x_i)) = sum(E(x_i)) =
= n_blocks * ((1 - 1/n_blocks)^n_rows) =
~= n_blocks * exp(-n_rows/n_blocks).
E(n_busy_blocks) = n_blocks*(1 - (1 - 1/n_blocks)^n_rows) =
~= n_blocks * (1 - exp(-n_rows/n_blocks)).
Average size of "hole" between neighbor non-empty blocks is
E(hole_size) = n_blocks/E(n_busy_blocks).
The total cost of reading all needed blocks in one "sweep" is:
E(n_busy_blocks) * disk_seek_cost(n_blocks/E(n_busy_blocks))
This cost estimate is calculated in get_sweep_read_cost().
3. Cost of Unique use is calculated in Unique::get_use_cost function.
ROR-union cost is calculated in the same way index_merge, but instead of
Unique a priority queue is used.
RETURN
Created read plan
NULL - Out of memory or no read scan could be built.
*/
