<listitem>
<para>
Sets the planner's assumption about the effective size of the
- disk cache that is available to a single query. The default
- setting of -1 selects a size equal to four times the size of <xref
- linkend="guc-shared-buffers">, but not less than the size of one
- shared buffer page, typically <literal>8kB</literal>. This value
- can be set manually if the automatic choice is too large or too
- small.
- </para>
-
- <para>
- This value is factored into estimates of the cost of using an index;
- a higher value makes it more likely index scans will be used, a
+ disk cache that is available to a single query. This is
+ factored into estimates of the cost of using an index; a
+ higher value makes it more likely index scans will be used, a
lower value makes it more likely sequential scans will be
used. When setting this parameter you should consider both
<productname>PostgreSQL</productname>'s shared buffers and the
memory allocated by <productname>PostgreSQL</productname>, nor
does it reserve kernel disk cache; it is used only for estimation
purposes. The system also does not assume data remains in
- the disk cache between queries. The auto-tuning
- selected by the default setting of -1 should give reasonable
- results if this database cluster can utilize most of the memory
- on this server.
+ the disk cache between queries.
+ </para>
+
+ <para>
+ If <varname>effective_cache_size</> is set to -1, which is the
+ default, the value is replaced by an automatically selected value,
+ currently four times the size of <xref linkend="guc-shared-buffers">.
+ For recommended settings of <varname>shared_buffers</>, this should
+ give reasonable results if this database cluster can use most of the
+ memory on the server.
</para>
</listitem>
</varlistentry>
#ifdef _MSC_VER
#include <float.h> /* for _isnan */
#endif
+#include <limits.h>
#include <math.h>
#include "access/htup_details.h"
#define LOG2(x) (log(x) / 0.693147180559945)
+
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;
-int effective_cache_size = -1;
+int effective_cache_size = -1; /* will get replaced */
Cost disable_cost = 1.0e10;
path->path.total_cost = startup_cost + run_cost;
}
-void
-set_default_effective_cache_size(void)
-{
- /*
- * If the value of effective_cache_size is -1, use the preferred
- * auto-tune value.
- */
- if (effective_cache_size == -1)
- {
- char buf[32];
-
- snprintf(buf, sizeof(buf), "%d", NBuffers * DEFAULT_EFFECTIVE_CACHE_SIZE_MULTI);
- SetConfigOption("effective_cache_size", buf, PGC_POSTMASTER, PGC_S_OVERRIDE);
- }
- Assert(effective_cache_size > 0);
-}
-
-/*
- * GUC check_hook for effective_cache_size
- */
-bool
-check_effective_cache_size(int *newval, void **extra, GucSource source)
-{
- /*
- * -1 indicates a request for auto-tune.
- */
- if (*newval == -1)
- {
- /*
- * If we haven't yet changed the boot_val default of -1, just let it
- * be. We'll fix it later.
- */
- if (effective_cache_size == -1)
- return true;
-
- /* Otherwise, substitute the auto-tune value */
- *newval = NBuffers * DEFAULT_EFFECTIVE_CACHE_SIZE_MULTI;
- }
-
- /* set minimum? */
- if (*newval < 1)
- *newval = 1;
-
- return true;
-}
-
/*
* index_pages_fetched
* Estimate the number of pages actually fetched after accounting for
{
return ceil(relation_byte_size(tuples, width) / BLCKSZ);
}
+
+/*
+ * GUC check_hook for effective_cache_size
+ */
+bool
+check_effective_cache_size(int *newval, void **extra, GucSource source)
+{
+ /*
+ * -1 is the documented way of requesting auto-tune, but we also treat
+ * zero as meaning that, since we don't consider zero a valid setting.
+ */
+ if (*newval <= 0)
+ {
+ /*
+ * If we haven't yet changed the initial default of -1, just let it
+ * be. We'll fix it later on during GUC initialization, when
+ * set_default_effective_cache_size is called. (If we try to do it
+ * immediately, we may not be looking at the final value of NBuffers.)
+ */
+ if (effective_cache_size == -1)
+ return true;
+
+ /*
+ * Otherwise, substitute the auto-tune value, being wary of overflow.
+ */
+ if (NBuffers < INT_MAX / 4)
+ *newval = NBuffers * 4;
+ else
+ *newval = INT_MAX;
+ }
+
+ Assert(*newval > 0);
+
+ return true;
+}
+
+/*
+ * initialize effective_cache_size at the end of GUC startup
+ */
+void
+set_default_effective_cache_size(void)
+{
+ /*
+ * If the value of effective_cache_size is still -1 (or zero), replace it
+ * with the auto-tune value.
+ */
+ if (effective_cache_size <= 0)
+ {
+ /* disable the short-circuit in check_effective_cache_size */
+ effective_cache_size = 0;
+ /* and let check_effective_cache_size() compute the setting */
+ SetConfigOption("effective_cache_size", "-1",
+ PGC_POSTMASTER, PGC_S_OVERRIDE);
+ }
+ Assert(effective_cache_size > 0);
+}
projects / postgresql.git / commitdiff