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Commit7a7ba33

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committed
Clean up some bogosities in path cost estimation, like
sometimes estimating an index scan of a table to be cheaper than asequential scan of the same tuples...
1 parent11a0027 commit7a7ba33

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1 file changed

+49
-28
lines changed

1 file changed

+49
-28
lines changed

‎src/backend/optimizer/path/costsize.c

Lines changed: 49 additions & 28 deletions
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,7 @@
77
*
88
*
99
* IDENTIFICATION
10-
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.34 1999/04/05 02:07:07 tgl Exp $
10+
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.35 1999/04/30 04:01:44 tgl Exp $
1111
*
1212
*-------------------------------------------------------------------------
1313
*/
@@ -129,28 +129,36 @@ cost_index(Oid indexid,
129129
intindextuples,
130130
boolis_injoin)
131131
{
132-
Costtemp;
133-
doubletemp2;
134-
135-
temp= (Cost)0;
132+
Costtemp=0;
136133

137134
if (!_enable_indexscan_&& !is_injoin)
138135
temp+=_disable_cost_;
139136

137+
/* We want to be sure we estimate the cost of an index scan
138+
* as more than the cost of a sequential scan (when selec == 1.0),
139+
* even if we don't have good stats. So, disbelieve zero index size.
140+
*/
141+
if (expected_indexpages <=0)
142+
expected_indexpages=1;
143+
if (indextuples <=0)
144+
indextuples=1;
145+
140146
/* expected index relation pages */
141147
temp+=expected_indexpages;
142148

143-
/* expected base relation pages */
144-
temp2= (reltuples==0) ? (double)0 : (double)relpages /reltuples;
145-
temp2=temp2* (double)selec*indextuples;
146-
147-
temp+=Min(relpages, (int)ceil(temp2));
149+
/* expected base relation pages
150+
* XXX this isn't really right, since we will access the table
151+
* nonsequentially and might have to fetch the same page
152+
* more than once. This calculation assumes the buffer cache
153+
* will prevent that from happening...
154+
*/
155+
temp+=ceil(((double)selec)* ((double)relpages));
148156

149157
/* per index tuples */
150-
temp=temp+ (_cpu_index_page_wight_*selec*indextuples);
158+
temp+=_cpu_index_page_wight_*selec*indextuples;
151159

152160
/* per heap tuples */
153-
temp=temp+ (_cpu_page_wight_*selec*reltuples);
161+
temp+=_cpu_page_wight_*selec*reltuples;
154162

155163
Assert(temp >=0);
156164
returntemp;
@@ -168,8 +176,13 @@ cost_index(Oid indexid,
168176
* 'pathkeys' is a list of sort keys
169177
* 'tuples' is the number of tuples in the relation
170178
* 'width' is the average tuple width in bytes
171-
* 'noread' is a flag indicating that the sort result can remain on disk
172-
*(i.e., the sort result is the result relation)
179+
* 'noread' is a flag indicating that the cost of reading the sort
180+
*source data should not be included (i.e., the caller
181+
*will account for it separately).
182+
*
183+
* NOTE: some callers currently pass NULL for pathkeys because they
184+
* can't conveniently supply sort keys. Since this routine doesn't
185+
* currently do anything with pathkeys anyway, that doesn't matter...
173186
*
174187
* Returns a flonum.
175188
*
@@ -179,28 +192,33 @@ cost_sort(List *pathkeys, int tuples, int width, bool noread)
179192
{
180193
Costtemp=0;
181194
intnpages=page_size(tuples,width);
182-
Costpages= (Cost)npages;
183-
CostnumTuples=tuples;
195+
doublelog_npages;
184196

185197
if (!_enable_sort_)
186198
temp+=_disable_cost_;
187-
if (tuples==0||pathkeys==NULL)
188-
{
189-
Assert(temp >=0);
190-
returntemp;
191-
}
192-
temp+=pages*base_log((double)pages, (double)2.0);
199+
200+
/* We want to be sure the cost of a sort is never estimated as zero,
201+
* even if passed-in tuple count is zero. Besides, mustn't do log(0)...
202+
*/
203+
if (npages <=0)
204+
npages=1;
205+
206+
log_npages=ceil(base_log((double)npages,2.0));
207+
if (log_npages <=0.0)
208+
log_npages=1.0;
209+
210+
temp+=npages*log_npages;
193211

194212
/*
195213
* could be base_log(pages, NBuffers), but we are only doing 2-way
196214
* merges
197215
*/
198-
temp+=_cpu_page_wight_*numTuples*
199-
base_log((double)pages, (double)2.0);
216+
temp+=_cpu_page_wight_*tuples*log_npages;
200217

201218
if (!noread)
202-
temp=temp+cost_seqscan(_NONAME_RELATION_ID_,npages,tuples);
203-
Assert(temp >=0);
219+
temp+=cost_seqscan(_NONAME_RELATION_ID_,npages,tuples);
220+
221+
Assert(temp>0);
204222

205223
returntemp;
206224
}
@@ -290,9 +308,12 @@ cost_mergejoin(Cost outercost,
290308

291309
temp+=outercost;
292310
temp+=innercost;
293-
temp+=cost_sort(outersortkeys,outersize,outerwidth, false);
294-
temp+=cost_sort(innersortkeys,innersize,innerwidth, false);
311+
if (outersortkeys)/* do we need to sort? */
312+
temp+=cost_sort(outersortkeys,outersize,outerwidth, true);
313+
if (innersortkeys)/* do we need to sort? */
314+
temp+=cost_sort(innersortkeys,innersize,innerwidth, true);
295315
temp+=_cpu_page_wight_* (outersize+innersize);
316+
296317
Assert(temp >=0);
297318

298319
returntemp;

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