MDEV-38975: HEAP engine BLOB/TEXT column support

Allow BLOB/TEXT columns in MEMORY (HEAP) engine tables by storing
blob data in variable-length continuation record chains within the
existing `HP_BLOCK` structure.

**Continuation runs**: blob data is split across contiguous sequences
of `recbuffer`-sized records. Each run stores a 10-byte header
(`next_cont` pointer + `run_rec_count`) in the first record; subsequent
records carry pure payload. Runs are linked via `next_cont` pointers.
Individual runs are capped at 65,535 records (`uint16` format limit);
larger blobs are automatically split into multiple runs.

**Free list scavenging**: on insert, the free list is walked read-only
(peek) tracking contiguous groups in descending address order (LIFO).
Qualifying groups (>= `min_run_records`) are unlinked and used. The
first non-qualifying group terminates the scan — remaining data is
allocated from the block tail. The free list is never disturbed when
no qualifying group is found.

**Record counting**: new `HP_SHARE::total_records` tracks all physical
records (primary + continuation). `HP_SHARE::records` remains logical
(primary-only) to preserve linear hash bucket mapping correctness.

**Reassembly**: per-handle `HP_INFO::blob_buff` reassembles blob data
on read, following the same pattern as InnoDB's `blob_heap`. Freed on
`heap_reset()` / `HA_EXTRA_NORMAL` / `heap_close()`.

**Hash functions**: `hp_rec_hashnr()`, `hp_rec_key_cmp()`, `hp_key_cmp()`,
`hp_make_key()` updated to handle `HA_BLOB_PART` key segments — reading
actual blob data via pointer dereference or chain materialization.

**SQL layer**: `choose_engine()` no longer rejects HEAP for blob tables
(replaced `blob_fields` check with `reclength > HA_MAX_REC_LENGTH`).
`remove_duplicates()` routes HEAP+blob to `remove_dup_with_compare()`.
`ha_heap::remember_rnd_pos()` / `restart_rnd_next()` implemented for
DISTINCT deduplication support.

Author: arcivanov

include/heap.h

@@ -131,21 +131,27 @@ typedef struct st_hp_keydef		/* Key definition with open */
   uint (*get_key_length)(struct st_hp_keydef *keydef, const uchar *key);
 } HP_KEYDEF;
 
+typedef struct st_hp_blob_desc
+{
+  uint offset;       /* Byte offset of blob descriptor within record buffer */
+  uint packlength;   /* 1, 2, 3, or 4: length prefix size */
+} HP_BLOB_DESC;
+
 typedef struct st_heap_share
 {
   HP_BLOCK block;
   HP_KEYDEF  *keydef;
   ulonglong data_length,index_length,max_table_size;
   ulonglong auto_increment;
   ulong min_records,max_records;	/* Params to open */
-  ulong records;			/* records */
+  ulong records;			/* Logical (primary) record count */
   ulong blength;			/* records rounded up to 2^n */
   ulong deleted;			/* Deleted records in database */
   uint key_stat_version;                /* version to indicate insert/delete */
   uint key_version;                     /* Updated on key change */
   uint file_version;                    /* Update on clear */
   uint reclength;			/* Length of one record */
-  uint visible;                         /* Offset to the visible/deleted mark */
+  uint visible;                         /* Offset to the flags byte (active/deleted/continuation) */
   uint changed;
   uint keys,max_key_length;
   uint currently_disabled_keys;    /* saved value from "keys" when disabled */
@@ -156,6 +162,9 @@ typedef struct st_heap_share
   THR_LOCK lock;
   my_bool delete_on_close;
   my_bool internal;                     /* Internal temporary table */
+  HP_BLOB_DESC *blob_descs;    /* Array of blob column descriptors */
+  uint blob_count;             /* Number of blob columns */
+  ulong total_records;         /* All active records (primary + blob continuation) */
   LIST open_list;
   uint auto_key;
   uint auto_key_type;			/* real type of the auto key segment */
@@ -181,6 +190,8 @@ typedef struct st_heap_info
   uint file_version;                    /* Version at scan */
   uint lastkey_len;
   my_bool implicit_emptied;
+  uchar *blob_buff;            /* Reassembly buffer for blob reads */
+  uint32 blob_buff_len;        /* Current allocated size of blob_buff */
   THR_LOCK_DATA lock;
   LIST open_list;
 } HP_INFO;
@@ -204,6 +215,8 @@ typedef struct st_heap_create_info
     open_count to 1. Is only looked at if not internal_table.
   */
   my_bool pin_share;
+  HP_BLOB_DESC *blob_descs;
+  uint blob_count;
 } HP_CREATE_INFO;
 
 	/* Prototypes for heap-functions */

mysql-test/main/cte_recursive.test

@@ -3212,6 +3212,8 @@ show create table t2;
 --eval insert ignore into t2 $query;
 drop table t2;
 set @@sql_mode="";
+# Rows with identical (level, mid) due to overflow have non-deterministic order
+--sorted_result
 --eval $query
 --eval create table t2 as $query;
 show create table t2;

mysql-test/main/distinct.result

@@ -1189,7 +1189,7 @@ insert into t1 values (1, 'Aa123456', 'abc'), (2, 'Bb7897777', 'def'),
 (3, 'Cc01287', 'xyz'), (5, 'd12345', 'efg');
 select distinct if(sum(a), b, 0) from t1 group by value(c) with rollup;
 if(sum(a), b, 0)
-Aa123456
+SOME_B_VALUE
 drop table t1;
 #
 # end of 10.5 tests

mysql-test/main/distinct.test

@@ -915,6 +915,9 @@ create table t1 (a int, b longtext, c varchar(18));
 insert into t1 values (1, 'Aa123456', 'abc'), (2, 'Bb7897777', 'def'),
   (3, 'Cc01287', 'xyz'), (5, 'd12345', 'efg');
 
+# ROLLUP row's b value is indeterminate (depends on last group processed),
+# which varies by temp table engine (HEAP vs Aria). Mask the value.
+--replace_regex /(Aa123456|Bb7897777|Cc01287|d12345)/SOME_B_VALUE/
 select distinct if(sum(a), b, 0) from t1 group by value(c) with rollup;
 drop table t1;
 

mysql-test/main/group_by.result

@@ -2510,10 +2510,10 @@ SELECT f3, MIN(f2) FROM t1 GROUP BY f1 LIMIT 1;
 f3	MIN(f2)
 blob	NULL
 DROP TABLE t1;
-the value below *must* be 1
+the value below *must* be 0 (HEAP supports blobs)
 show status like 'Created_tmp_disk_tables';
 Variable_name	Value
-Created_tmp_disk_tables	1
+Created_tmp_disk_tables	0
 #
 #  Bug #1002146: Unneeded filesort if usage of join buffer is not allowed
 #  (bug mdev-645)

mysql-test/main/group_by.test

@@ -1671,14 +1671,14 @@ DROP TABLE t1, t2;
 --disable_ps2_protocol
 --disable_view_protocol
 --disable_cursor_protocol
-FLUSH STATUS; # this test case *must* use Aria temp tables
+FLUSH STATUS;
 
 CREATE TABLE t1 (f1 INT, f2 decimal(20,1), f3 blob);
 INSERT INTO t1 values(11,NULL,'blob'),(11,NULL,'blob');
 SELECT f3, MIN(f2) FROM t1 GROUP BY f1 LIMIT 1;
 DROP TABLE t1;
 
---echo the value below *must* be 1
+--echo the value below *must* be 0 (HEAP supports blobs)
 show status like 'Created_tmp_disk_tables';
 --enable_cursor_protocol
 --enable_view_protocol

mysql-test/main/select.result

@@ -576,18 +576,18 @@ bedlam	1
 bedpost	1
 boasted	1
 set tmp_memory_table_size=default;
-select distinct fld3,repeat("a",length(fld3)),count(*) from t2 group by companynr,fld3 limit 100,10;
+select distinct fld3,repeat("a",length(fld3)),count(*) from t2 group by companynr,fld3 order by fld3 limit 100,10;
 fld3	repeat("a",length(fld3))	count(*)
-circus	aaaaaa	1
-cited	aaaaa	1
-Colombo	aaaaaaa	1
-congresswoman	aaaaaaaaaaaaa	1
-contrition	aaaaaaaaaa	1
-corny	aaaaa	1
-cultivation	aaaaaaaaaaa	1
-definiteness	aaaaaaaaaaaa	1
-demultiplex	aaaaaaaaaaa	1
-disappointing	aaaaaaaaaaaaa	1
+Baird	aaaaa	1
+balled	aaaaaa	1
+ballgown	aaaaaaaa	1
+Baltimorean	aaaaaaaaaaa	1
+bankruptcies	aaaaaaaaaaaa	1
+Barry	aaaaa	1
+batting	aaaaaaa	1
+beaner	aaaaaa	1
+beasts	aaaaaa	1
+beaters	aaaaaaa	1
 select distinct companynr,rtrim(space(512+companynr)) from t3 order by 1,2;
 companynr	rtrim(space(512+companynr))
 37	

mysql-test/main/select.test

@@ -1442,7 +1442,8 @@ select distinct fld3,count(*) from t2 group by companynr,fld3 limit 10;
 set tmp_memory_table_size=0; # force on-disk tmp table
 select distinct fld3,count(*) from t2 group by companynr,fld3 limit 10;
 set tmp_memory_table_size=default;
-select distinct fld3,repeat("a",length(fld3)),count(*) from t2 group by companynr,fld3 limit 100,10;
+# ORDER BY fld3 ensures deterministic LIMIT window regardless of temp table engine
+select distinct fld3,repeat("a",length(fld3)),count(*) from t2 group by companynr,fld3 order by fld3 limit 100,10;
 
 #
 # A big order by that should trigger a merge in filesort

mysql-test/suite/heap/blob_dedup.result

@@ -0,0 +1,15 @@
+CREATE TABLE t1 (a mediumtext) ENGINE=HEAP;
+INSERT INTO t1 VALUES ('abc'),('def');
+SELECT DISTINCT a FROM t1;
+a
+abc
+def
+DROP TABLE t1;
+CREATE TABLE t1 (a mediumtext);
+CREATE TABLE t2 (b varchar(20));
+INSERT INTO t1 VALUES ('a'),('b');
+SELECT left(a,100000000) FROM t1 UNION SELECT b FROM t2;
+left(a,100000000)
+a
+b
+DROP TABLE t1, t2;

mysql-test/suite/heap/blob_dedup.test

@@ -0,0 +1,10 @@
+CREATE TABLE t1 (a mediumtext) ENGINE=HEAP;
+INSERT INTO t1 VALUES ('abc'),('def');
+SELECT DISTINCT a FROM t1;
+DROP TABLE t1;
+
+CREATE TABLE t1 (a mediumtext);
+CREATE TABLE t2 (b varchar(20));
+INSERT INTO t1 VALUES ('a'),('b');
+SELECT left(a,100000000) FROM t1 UNION SELECT b FROM t2;
+DROP TABLE t1, t2;