Reduce memory usage of tsvector type analyze function.

compute_tsvector_stats() detoasted and kept in memory every tsvector value
in the sample, but that can be a lot of memory. The original bug report
described a case using over 10 gigabytes, with statistics target of 10000
(the maximum).

To fix, allocate a separate copy of just the lexemes that we keep around,
and free the detoasted tsvector values as we go. This adds some palloc/pfree
overhead, when you have a lot of distinct lexemes in the sample, but it's
better than running out of memory.

Fixes bug #14654 reported by James C. Reviewed by Tom Lane. Backport to
all supported versions.

Discussion: https://www.postgresql.org/message-id/20170514200602.1451.46797@wrigleys.postgresql.org

diff --git a/src/backend/tsearch/ts_typanalyze.c b/src/backend/tsearch/ts_typanalyze.c
index 817453ce0115a0bb5e5b1427f86524c5f074af4b..fd89ea0eac0208c2eaed449839fe39d0083b3e23 100644 (file)
--- a/src/backend/tsearch/ts_typanalyze.c
+++ b/src/backend/tsearch/ts_typanalyze.c
@@ -232,9 +232,7 @@ compute_tsvector_stats(VacAttrStats *stats,
 
        /*
         * We loop through the lexemes in the tsvector and add them to our
-        * tracking hashtable.  Note: the hashtable entries will point into
-        * the (detoasted) tsvector value, therefore we cannot free that
-        * storage until we're done.
+        * tracking hashtable.
         */
        lexemesptr = STRPTR(vector);
        curentryptr = ARRPTR(vector);
@@ -242,7 +240,12 @@ compute_tsvector_stats(VacAttrStats *stats,
        {
            bool        found;
 
-           /* Construct a hash key */
+           /*
+            * Construct a hash key.  The key points into the (detoasted)
+            * tsvector value at this point, but if a new entry is created, we
+            * make a copy of it.  This way we can free the tsvector value
+            * once we've processed all its lexemes.
+            */
            hash_key.lexeme = lexemesptr + curentryptr->pos;
            hash_key.length = curentryptr->len;
 
@@ -261,6 +264,9 @@ compute_tsvector_stats(VacAttrStats *stats,
                /* Initialize new tracking list element */
                item->frequency = 1;
                item->delta = b_current - 1;
+
+               item->key.lexeme = palloc(hash_key.length);
+               memcpy(item->key.lexeme, hash_key.lexeme, hash_key.length);
            }
 
            /* lexeme_no is the number of elements processed (ie N) */
@@ -276,6 +282,10 @@ compute_tsvector_stats(VacAttrStats *stats,
            /* Advance to the next WordEntry in the tsvector */
            curentryptr++;
        }
+
+       /* If the vector was toasted, free the detoasted copy. */
+       if (TSVectorGetDatum(vector) != value)
+           pfree(vector);
    }
 
    /* We can only compute real stats if we found some non-null values. */
@@ -447,9 +457,12 @@ prune_lexemes_hashtable(HTAB *lexemes_tab, int b_current)
    {
        if (item->frequency + item->delta <= b_current)
        {
+           char       *lexeme = item->key.lexeme;
+
            if (hash_search(lexemes_tab, (const void *) &item->key,
                            HASH_REMOVE, NULL) == NULL)
                elog(ERROR, "hash table corrupted");
+           pfree(lexeme);
        }
    }
 }