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Cause planner to make use of average-column-width statistic that is now

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collected by ANALYZE.  Also, add some modest amount of intelligence to
guesses that are used for varlena columns in the absence of any ANALYZE
statistics.  The 'width' reported by EXPLAIN is finally something less
than totally bogus for varlena columns ... and, in consequence, hashjoin
estimating should be a little better ...
This commit is contained in:
Tom Lane
2001-05-09 00:35:09 +00:00
parent b66a0cca57
commit 6cda3ad8fe
4 changed files with 124 additions and 42 deletions
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78
src/backend/utils/cache/lsyscache.c vendored
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@ -7,7 +7,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/utils/cache/lsyscache.c,v 1.53 2001/05/07 00:43:24 tgl Exp $
* $Header: /cvsroot/pgsql/src/backend/utils/cache/lsyscache.c,v 1.54 2001/05/09 00:35:09 tgl Exp $
*
* NOTES
* Eventually, the index information should go through here, too.
@ -749,6 +749,56 @@ get_typdefault(Oid typid)
return returnValue;
}
/*
* get_typavgwidth
*
* Given a type OID and a typmod value (pass -1 if typmod is unknown),
* estimate the average width of values of the type. This is used by
* the planner, which doesn't require absolutely correct results;
* it's OK (and expected) to guess if we don't know for sure.
*/
int32
get_typavgwidth(Oid typid, int32 typmod)
{
int typlen = get_typlen(typid);
int32 maxwidth;
/*
* Easy if it's a fixed-width type
*/
if (typlen > 0)
return typlen;
/*
* type_maximum_size knows the encoding of typmod for some datatypes;
* don't duplicate that knowledge here.
*/
maxwidth = type_maximum_size(typid, typmod);
if (maxwidth > 0)
{
/*
* For BPCHAR, the max width is also the only width. Otherwise
* we need to guess about the typical data width given the max.
* A sliding scale for percentage of max width seems reasonable.
*/
if (typid == BPCHAROID)
return maxwidth;
if (maxwidth <= 32)
return maxwidth; /* assume full width */
if (maxwidth < 1000)
return 32 + (maxwidth - 32) / 2; /* assume 50% */
/*
* Beyond 1000, assume we're looking at something like
* "varchar(10000)" where the limit isn't actually reached often,
* and use a fixed estimate.
*/
return 32 + (1000 - 32) / 2;
}
/*
* Ooops, we have no idea ... wild guess time.
*/
return 32;
}
/*
* get_typtype
*
@ -782,6 +832,32 @@ get_typtype(Oid typid)
/* ---------- STATISTICS CACHE ---------- */
/*
* get_attavgwidth
*
* Given the table and attribute number of a column, get the average
* width of entries in the column. Return zero if no data available.
*/
int32
get_attavgwidth(Oid relid, AttrNumber attnum)
{
HeapTuple tp;
tp = SearchSysCache(STATRELATT,
ObjectIdGetDatum(relid),
Int16GetDatum(attnum),
0, 0);
if (HeapTupleIsValid(tp))
{
int32 stawidth = ((Form_pg_statistic) GETSTRUCT(tp))->stawidth;
ReleaseSysCache(tp);
if (stawidth > 0)
return stawidth;
}
return 0;
}
/*
* get_attstatsslot
*