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Add a GUC parameter seq_page_cost, and use that everywhere we formerly

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assumed that a sequential page fetch has cost 1.0.  This patch doesn't
in itself change the system's behavior at all, but it opens the door to
people adopting other units of measurement for EXPLAIN costs.  Also, if
we ever decide it's worth inventing per-tablespace access cost settings,
this change provides a workable intellectual framework for that.
This commit is contained in:
Tom Lane
2006-06-05 02:49:58 +00:00
parent a837851dc0
commit eed6c9ed7e
8 changed files with 243 additions and 187 deletions
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83
src/backend/optimizer/path/costsize.c
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@ -3,14 +3,19 @@
* costsize.c
* Routines to compute (and set) relation sizes and path costs
*
* Path costs are measured in units of disk accesses: one sequential page
* fetch has cost 1. All else is scaled relative to a page fetch, using
* the scaling parameters
* Path costs are measured in arbitrary units established by these basic
* parameters:
*
* seq_page_cost Cost of a sequential page fetch
* random_page_cost Cost of a non-sequential page fetch
* cpu_tuple_cost Cost of typical CPU time to process a tuple
* cpu_index_tuple_cost Cost of typical CPU time to process an index tuple
* cpu_operator_cost Cost of CPU time to process a typical WHERE operator
* cpu_operator_cost Cost of CPU time to execute an operator or function
*
* We expect that the kernel will typically do some amount of read-ahead
* optimization; this in conjunction with seek costs means that seq_page_cost
* is normally considerably less than random_page_cost. (However, if the
* database is fully cached in RAM, it is reasonable to set them equal.)
*
* We also use a rough estimate "effective_cache_size" of the number of
* disk pages in Postgres + OS-level disk cache. (We can't simply use
@ -49,7 +54,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.155 2006/03/05 15:58:28 momjian Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.156 2006/06/05 02:49:58 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -85,12 +90,14 @@
(path)->parent->rows)
double effective_cache_size = DEFAULT_EFFECTIVE_CACHE_SIZE;
double seq_page_cost = DEFAULT_SEQ_PAGE_COST;
double random_page_cost = DEFAULT_RANDOM_PAGE_COST;
double cpu_tuple_cost = DEFAULT_CPU_TUPLE_COST;
double cpu_index_tuple_cost = DEFAULT_CPU_INDEX_TUPLE_COST;
double cpu_operator_cost = DEFAULT_CPU_OPERATOR_COST;
double effective_cache_size = DEFAULT_EFFECTIVE_CACHE_SIZE;
Cost disable_cost = 100000000.0;
bool enable_seqscan = true;
@ -156,14 +163,8 @@ cost_seqscan(Path *path, PlannerInfo *root,
/*
* disk costs
*
* The cost of reading a page sequentially is 1.0, by definition. Note
* that the Unix kernel will typically do some amount of read-ahead
* optimization, so that this cost is less than the true cost of reading a
* page from disk. We ignore that issue here, but must take it into
* account when estimating the cost of non-sequential accesses!
*/
run_cost += baserel->pages; /* sequential fetches with cost 1.0 */
run_cost += seq_page_cost * baserel->pages;
/* CPU costs */
startup_cost += baserel->baserestrictcost.startup;
@ -194,20 +195,21 @@ cost_seqscan(Path *path, PlannerInfo *root,
* with the entirely ad-hoc equations (writing relsize for
* relpages/effective_cache_size):
* if relsize >= 1:
* random_page_cost - (random_page_cost-1)/2 * (1/relsize)
* random_page_cost - (random_page_cost-seq_page_cost)/2 * (1/relsize)
* if relsize < 1:
* 1 + ((random_page_cost-1)/2) * relsize ** 2
* These give the right asymptotic behavior (=> 1.0 as relpages becomes
* small, => random_page_cost as it becomes large) and meet in the middle
* with the estimate that the cache is about 50% effective for a relation
* of the same size as effective_cache_size. (XXX this is probably all
* wrong, but I haven't been able to find any theory about how effective
* seq_page_cost + ((random_page_cost-seq_page_cost)/2) * relsize ** 2
* These give the right asymptotic behavior (=> seq_page_cost as relpages
* becomes small, => random_page_cost as it becomes large) and meet in the
* middle with the estimate that the cache is about 50% effective for a
* relation of the same size as effective_cache_size. (XXX this is probably
* all wrong, but I haven't been able to find any theory about how effective
* a disk cache should be presumed to be.)
*/
static Cost
cost_nonsequential_access(double relpages)
{
double relsize;
double random_delta;
/* don't crash on bad input data */
if (relpages <= 0.0 || effective_cache_size <= 0.0)
@ -215,19 +217,17 @@ cost_nonsequential_access(double relpages)
relsize = relpages / effective_cache_size;
random_delta = (random_page_cost - seq_page_cost) * 0.5;
if (relsize >= 1.0)
return random_page_cost - (random_page_cost - 1.0) * 0.5 / relsize;
return random_page_cost - random_delta / relsize;
else
return 1.0 + (random_page_cost - 1.0) * 0.5 * relsize * relsize;
return seq_page_cost + random_delta * relsize * relsize;
}
/*
* cost_index
* Determines and returns the cost of scanning a relation using an index.
*
* NOTE: an indexscan plan node can actually represent several passes,
* but here we consider the cost of just one pass.
*
* 'index' is the index to be used
* 'indexQuals' is the list of applicable qual clauses (implicit AND semantics)
* 'is_injoin' is T if we are considering using the index scan as the inside
@ -327,9 +327,9 @@ cost_index(IndexPath *path, PlannerInfo *root,
* be just sT. What's more, these will be sequential fetches, not the
* random fetches that occur in the uncorrelated case. So, depending on
* the extent of correlation, we should estimate the actual I/O cost
* somewhere between s * T * 1.0 and PF * random_cost. We currently
* interpolate linearly between these two endpoints based on the
* correlation squared (XXX is that appropriate?).
* somewhere between s * T * seq_page_cost and PF * random_page_cost.
* We currently interpolate linearly between these two endpoints based on
* the correlation squared (XXX is that appropriate?).
*
* In any case the number of tuples fetched is Ns.
*----------
@ -346,8 +346,10 @@ cost_index(IndexPath *path, PlannerInfo *root,
{
pages_fetched =
(2.0 * T * tuples_fetched) / (2.0 * T + tuples_fetched);
if (pages_fetched > T)
if (pages_fetched >= T)
pages_fetched = T;
else
pages_fetched = ceil(pages_fetched);
}
else
{
@ -364,6 +366,7 @@ cost_index(IndexPath *path, PlannerInfo *root,
pages_fetched =
b + (tuples_fetched - lim) * (T - b) / T;
}
pages_fetched = ceil(pages_fetched);
}
/*
@ -373,7 +376,7 @@ cost_index(IndexPath *path, PlannerInfo *root,
* rather than using cost_nonsequential_access, since we've already
* accounted for caching effects by using the Mackert model.
*/
min_IO_cost = ceil(indexSelectivity * T);
min_IO_cost = ceil(indexSelectivity * T) * seq_page_cost;
max_IO_cost = pages_fetched * random_page_cost;
/*
@ -461,19 +464,21 @@ cost_bitmap_heap_scan(Path *path, PlannerInfo *root, RelOptInfo *baserel,
T = (baserel->pages > 1) ? (double) baserel->pages : 1.0;
pages_fetched = (2.0 * T * tuples_fetched) / (2.0 * T + tuples_fetched);
if (pages_fetched > T)
if (pages_fetched >= T)
pages_fetched = T;
else
pages_fetched = ceil(pages_fetched);
/*
* For small numbers of pages we should charge random_page_cost apiece,
* while if nearly all the table's pages are being read, it's more
* appropriate to charge 1.0 apiece. The effect is nonlinear, too. For
* lack of a better idea, interpolate like this to determine the cost per
* page.
* appropriate to charge seq_page_cost apiece. The effect is nonlinear,
* too. For lack of a better idea, interpolate like this to determine the
* cost per page.
*/
if (pages_fetched >= 2.0)
cost_per_page = random_page_cost -
(random_page_cost - 1.0) * sqrt(pages_fetched / T);
(random_page_cost - seq_page_cost) * sqrt(pages_fetched / T);
else
cost_per_page = random_page_cost;
@ -833,9 +838,9 @@ cost_sort(Path *path, PlannerInfo *root,
else
log_runs = 1.0;
npageaccesses = 2.0 * npages * log_runs;
/* Assume half are sequential (cost 1), half are not */
/* Assume half are sequential, half are not */
startup_cost += npageaccesses *
(1.0 + cost_nonsequential_access(npages)) * 0.5;
(seq_page_cost + cost_nonsequential_access(npages)) * 0.5;
}
/*
@ -871,8 +876,8 @@ cost_material(Path *path,
double npages = ceil(nbytes / BLCKSZ);
/* We'll write during startup and read during retrieval */
startup_cost += npages;
run_cost += npages;
startup_cost += seq_page_cost * npages;
run_cost += seq_page_cost * npages;
}
/*