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 ab224b76b868ede4dccf13384bf39cf40e393e4c..320c7f1a61649d081f8c8fa1ffc0a12a2c874309 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);
                }
        }
 }