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 <[email protected]>
Reviewed-by: Alvaro Herrera <[email protected]>
Reviewed-by: Tomas Vondra <[email protected]>
Reviewed-by: Thomas Munro <[email protected]>
Reviewed-by: David Rowley <[email protected]>
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 */