Remove large fill factor support from dynahash.c.

Since ancient times we have had support for a fill factor (maximum load
factor) to be set for a dynahash hash table, but:

1.  It was an integer, whereas for in-memory hash tables interesting
load factor targets are probably somewhere near the 0.75-1.0 range.

2.  It was implemented in a way that performed an expensive division
operation that regularly showed up in profiles.

3.  We are not aware of anyone ever having used a non-default value.

Therefore, remove support, effectively fixing it at 1.

Author: Jakub Wartak <Jakub.Wartak@tomtom.com>
Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/VI1PR0701MB696044FC35013A96FECC7AC8F62D0%40VI1PR0701MB6960.eurprd07.prod.outlook.com

diff --git a/src/backend/utils/hash/dynahash.c b/src/backend/utils/hash/dynahash.c
index f4fbccdd7e444df9b31dbb581bc172d08251e651..1122e2e5e59adeae5232dd3d75259a1a7b618656 100644 (file)
--- a/src/backend/utils/hash/dynahash.c
+++ b/src/backend/utils/hash/dynahash.c
@@ -122,7 +122,6 @@
 #define DEF_SEGSIZE                       256
 #define DEF_SEGSIZE_SHIFT         8    /* must be log2(DEF_SEGSIZE) */
 #define DEF_DIRSIZE                       256
-#define DEF_FFACTOR                       1    /* default fill factor */
 
 /* Number of freelists to be used for a partitioned hash table. */
 #define NUM_FREELISTS                  32
@@ -191,7 +190,6 @@ struct HASHHDR
        Size            keysize;                /* hash key length in bytes */
        Size            entrysize;              /* total user element size in bytes */
        long            num_partitions; /* # partitions (must be power of 2), or 0 */
-       long            ffactor;                /* target fill factor */
        long            max_dsize;              /* 'dsize' limit if directory is fixed size */
        long            ssize;                  /* segment size --- must be power of 2 */
        int                     sshift;                 /* segment shift = log2(ssize) */
@@ -497,8 +495,6 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
                /* ssize had better be a power of 2 */
                Assert(hctl->ssize == (1L << hctl->sshift));
        }
-       if (flags & HASH_FFACTOR)
-               hctl->ffactor = info->ffactor;
 
        /*
         * SHM hash tables have fixed directory size passed by the caller.
@@ -603,8 +599,6 @@ hdefault(HTAB *hashp)
 
        hctl->num_partitions = 0;       /* not partitioned */
 
-       hctl->ffactor = DEF_FFACTOR;
-
        /* table has no fixed maximum size */
        hctl->max_dsize = NO_MAX_DSIZE;
 
@@ -670,11 +664,10 @@ init_htab(HTAB *hashp, long nelem)
                        SpinLockInit(&(hctl->freeList[i].mutex));
 
        /*
-        * Divide number of elements by the fill factor to determine a desired
-        * number of buckets.  Allocate space for the next greater power of two
-        * number of buckets
+        * Allocate space for the next greater power of two number of buckets,
+        * assuming a desired maximum load factor of 1.
         */
-       nbuckets = next_pow2_int((nelem - 1) / hctl->ffactor + 1);
+       nbuckets = next_pow2_int(nelem);
 
        /*
         * In a partitioned table, nbuckets must be at least equal to
@@ -733,7 +726,6 @@ init_htab(HTAB *hashp, long nelem)
                        "DIRECTORY SIZE  ", hctl->dsize,
                        "SEGMENT SIZE    ", hctl->ssize,
                        "SEGMENT SHIFT   ", hctl->sshift,
-                       "FILL FACTOR     ", hctl->ffactor,
                        "MAX BUCKET      ", hctl->max_bucket,
                        "HIGH MASK       ", hctl->high_mask,
                        "LOW  MASK       ", hctl->low_mask,
@@ -761,7 +753,7 @@ hash_estimate_size(long num_entries, Size entrysize)
                                elementAllocCnt;
 
        /* estimate number of buckets wanted */
-       nBuckets = next_pow2_long((num_entries - 1) / DEF_FFACTOR + 1);
+       nBuckets = next_pow2_long(num_entries);
        /* # of segments needed for nBuckets */
        nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1);
        /* directory entries */
@@ -804,7 +796,7 @@ hash_select_dirsize(long num_entries)
                                nDirEntries;
 
        /* estimate number of buckets wanted */
-       nBuckets = next_pow2_long((num_entries - 1) / DEF_FFACTOR + 1);
+       nBuckets = next_pow2_long(num_entries);
        /* # of segments needed for nBuckets */
        nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1);
        /* directory entries */
@@ -975,7 +967,7 @@ hash_search_with_hash_value(HTAB *hashp,
                 * order of these tests is to try to check cheaper conditions first.
                 */
                if (!IS_PARTITIONED(hctl) && !hashp->frozen &&
-                       hctl->freeList[0].nentries / (long) (hctl->max_bucket + 1) >= hctl->ffactor &&
+                       hctl->freeList[0].nentries > (long) (hctl->max_bucket + 1) &&
                        !has_seq_scans(hashp))
                        (void) expand_table(hashp);
        }

diff --git a/src/include/utils/hsearch.h b/src/include/utils/hsearch.h
index f1deb9beab0458db9c0557c23edb9916226f7d27..bebf89b3c451fb638c7b9a49fa71440169c39609 100644 (file)
--- a/src/include/utils/hsearch.h
+++ b/src/include/utils/hsearch.h
@@ -68,7 +68,6 @@ typedef struct HASHCTL
        long            ssize;                  /* segment size */
        long            dsize;                  /* (initial) directory size */
        long            max_dsize;              /* limit to dsize if dir size is limited */
-       long            ffactor;                /* fill factor */
        Size            keysize;                /* hash key length in bytes */
        Size            entrysize;              /* total user element size in bytes */
        HashValueFunc hash;                     /* hash function */
@@ -83,7 +82,6 @@ typedef struct HASHCTL
 #define HASH_PARTITION 0x0001  /* Hashtable is used w/partitioned locking */
 #define HASH_SEGMENT   0x0002  /* Set segment size */
 #define HASH_DIRSIZE   0x0004  /* Set directory size (initial and max) */
-#define HASH_FFACTOR   0x0008  /* Set fill factor */
 #define HASH_ELEM              0x0010  /* Set keysize and entrysize */
 #define HASH_BLOBS             0x0020  /* Select support functions for binary keys */
 #define HASH_FUNCTION  0x0040  /* Set user defined hash function */